WASD Scripting Environment

2.CGI

The information in this chapter merely outlines the WASD implementation details, which are in general very much vanilla CGI and NCSA CGI (Common Gateway Interface) compliant, originally based the INTERNET-DRAFT authored by D.Robinson ([email protected]), 8 January 1996, confirmed against the final RFC 3875, authored by David Robinson ([email protected]) and Ken A.L.Coar ([email protected]), October 2004.

2.1CGI Environment Variables

With the standard CGI environment variables are provided to the script via DCL global symbols. Each CGI variable symbol name is prefixed with "WWW_" (by default, although this can be changed using the "/CGI_PREFIX" qualifier and the SET CGIPREFIX mapping rule, see "Features and Facilities", this is not recommended if the WASD VMS scripts are to be used, as they expect CGI variable symbols to be prefixed in this manner).

There are a number of non-"standard" CGI variables to assist in tailoring scripts for the WASD environment. Do not make your scripts dependent on any of these if portability is a goal.

CGI variable capacity now varies significantly with VMS version.

The total size of all CGI variable names and values is determined by the value of [BufferSizeDclCommand] configuration directive, which determines the total buffer space of a mailbox providing the script's SYS$COMMAND. The default value of 4096 bytes will be ample for the typical CGI script request, however if it contains very large individual variables or a large number of form fields, etc., it may be possible to exhaust this quantity.

VMS V7.3-2 and later …

CGI variables may contain values in excess of 8000 characters (the full 8192 symbol capacity cannot be realized due to the way the symbols are created via the CLI). This is a significant increase on earlier capacities. Mailbox buffer [BufferSizeDclCommand] may need to be increased if this capacity is to be fully utilized.

VMS V7.3-1 and earlier …

Values may contain approximately 1000 characters minus the size of the variable name. This should still be sufficient for most circumstances (if not consider using CGIplus or ISAPI, extensions to CGI programming which remove this limitation). Why such an odd number and why a little rubbery? A DCL command line with these versions is limited to 255 characters so the symbols for larger variables are built up over successive DCL commands with the limit determined by CLI behaviour.

Symbol Truncation

On VMS V7.3-2 and later symbol capacity should never be an issue (well, perhaps only with the most extraordinarily poorly designed script). With VMS V7.3-1 and earlier, with a symbol value that is too large, the server by default aborts the request, generating and returning a 500 HTTP status. Experience has shown that this occurs very rarely. If it does occur it is possible to instruct the server to instead truncate the CGI variable value and continue processing. Any CGI variable that is truncated in such a manner has its name placed in CGI variable SERVER_TRUNCATE, so that a script can check for, and take appropriate action on, any such truncation. To have the server truncate such variables instead of aborting processing SET the path using the script=symbol=truncate mapping rule. For example

CGI Variables

Remember, by default all variables are prefixed by "WWW_" (though this may be modified using the set CGIprefix= mapping rule), and not all variables will be present for all requests. These CGI environment variables reflect a combination of HTTP/1.1 and HTTP/1.0 request parameters.

CGI Environment Variables

If the request path is set to provide them, there are also be variables providing information about a Secure Sockets Layer transported request's SSL environment.

Query String Variables

In line with other CGI implementations, additional, non-compliant variables are provided to ease CGI interfacing. These provide the various components of any query string. A keyword query string and a form query string are parsed into

Variables named WWW_KEY_number will be generated if the query string contains one or more plus ("+") and no equate symbols ("=").

Variables named WWW_FORM_form-element-name will be generated if the query string contains one or more equate symbols. Generally such a query string is used to encode form-URL-encoded (MIME type x-www-form-urlencoded) requests. By default the server will report an incorrect encoding with a 400 error response. However some scripts use malformed encodings and so this behaviour may be suppressed using the set script=query=relaxed mapping rule.

To suppress this decoding completely (and save a few CPU cycles) use the following rule.

UNIQUE_ID Note

The UNIQUE_ID variable is a mostly Apache-compliant implementation (the "_" has been substituted for the "@" to allow its use in file names), for each request generating a globally and temporally unique 19 character string that can be used where such a identifier might be needed. This string contains only "A"-"Z", "a"-"z", "0"-"9", "_" and "-" characters and is generated using a combination of time-stamp, host IP address, server system process identifier and counter, and is "guaranteed" to be unique in (Internet) space and time.

VMS Apache (CSWS) Compliance

WASD v7.0 had its CGI environment tailored slightly to ease portability between VMS Apache (Compaq Secure Web Server) and WASD. This included the provision of an APACHE$INPUT: stream and several Apache-specific CGI variables (see the table below). The CGILIB C function library (1.12 Scripting Function Library) has also been made CSWS V1.0-1 and later (Apache 1.3.12 and higher) compliant.

CGI Variable Demonstration

The basic CGI symbol names are demonstrated here with a call to a script that simply executes the following DCL code:

The source code for this simple script is:

WASD_ROOT:[SRC.OTHER]CGI_SYMBOLS.COM

2.2Script Output

This information applies to all non-DECnet based scripting, CGI, CGIplus, RTE, ISAPI. WASD uses mailboxes for script inter-process communication (IPC). These are efficient, versatile and allow direct output from all VMS environments and utilities. Like many VMS record-oriented devices however there are some things to consider when using them (also see ‘IPC Tickler’ in 2.2.1 CGI Compliant Output).

• Record-Oriented

  The mailboxes are created record, not stream oriented. This means records output by standard VMS means (e.g. DCL, utilities, programming languages) are discretely identified and may be processed appropriately by the server as text or binary depending on the content-type.

• Maximum Record Size

  Being record oriented there is a maximum record size (MRS) that can be output. Records larger than this result in SYSTEM-F-MBTOOSML errors. The WASD default is 4096 bytes. This may be changed using the [BufferSizeDclOutput] configuration directive. This allocation consumes process BYTLM with each mailbox created so the account must be dimensioned sufficiently to supply demands for this quota. The maximum possible size for this is a VMS-limit of 60,000 bytes.

• Buffer Space

  When created the mailbox has its buffer space set. With WASD IPC mailboxes this is the same as the MRS. The total data buffered may not exceed this without the script entering a wait state (for the mailbox contents to be cleared by the server). As mailboxes use a little of the buffer space to delimit records stored in it the amount of data is actually less than the total buffer space.

To determine the maximum record size and total capacity of the mailbox buffer between server and script WASD provides a CGI environment variable, GATEWAY_MRS, containing an integer with this value.

2.2.1CGI Compliant Output

Script response may be CGI or NPH compliant (2.2.2 Non-Parsed-Header Output). CGI compliance means the script's response must begin with a line containing one of the following fields.

• Status: an HTTP status code and associated explanation string
• Content-Type: the script body's MIME content-type
• Location: a redirection URL (absolute or relative)

Other HTTP-compliant response fields may follow, with the response header terminated and the response body begun by a single empty line. The following are examples of CGI-compliant responses.

Strict CGI output compliance can be enabled and disabled using the [CgiStrictOutput] configuration directive. With it disabled the server will accept any output from the script, if not CGI or NPH compliant then it automatically generates plain-text header. When enabled, if not a CGI or NPH header the server returns a "502 Bad Gateway" error. For debugging scripts generating this error introduce a plain-text debug mode and header, or use the WATCH facility's CGI item (see Features and Facilities).

Output and HTTP/1.1

With HTTP/1.1 it is generally better to use CGI than NPH responses. A CGI response allows the server to parse the response header and from that make decisions about connection persistence and content-encoding. These can contribute significantly to reducing response latency and content transfer efficiency. It allows any policy established by server configuration for such characteristics to be employed.

WASD Specifics

This section describes how WASD deals with some particular output issues (also see ‘IPC Tickler’ in 2.2.1 CGI Compliant Output).

• Content-Type: text/…

  If the script response content-type is "text/…" (text document) WASD assumes that output will be line-oriented and requiring HTTP carriage-control (each record/line terminated by a line-feed), and will ensure each record it receives is correctly terminated before passing it to the client. In this way DCL procedure output (and the VMS environment in general) is supported transparently. Any other content-type is assumed to be binary and no carriage-control is enforced. This default behaviour may be modified as described below.

• Carriage-Control

  Carriage-control behaviour for any content-type may be explicitly set using either of two additional response header fields. The term stream is used to describe the server just transfering records, without additional processing, as they were received from the script. This is obviously necessary for binary/raw content such as images, octet-streams, etc. The term record describes the server ensuring each record it receives has correct carriage-control - a trailing newline. If not present one is added. This mode is useful for VMS textual streams (e.g. output from DCL and VMS utilities).

  Using the Apache Group's proposed CGI/1.2 "Script-Control:" field. The WASD extension-directives X-record-mode and X-stream-mode sets the script output into each of the respective modes (‘Script-Control:’ in 2.2.1 CGI Compliant Output).

  Examples of usage this field:

  Script-Control: X-stream-mode Script-Control: X-record-mode
• Script Output Buffering

  By default WASD writes each record received from the script to the client as it is received. This can range from a single byte to a complete mailbox buffer full. WASD leaves it up to the script to determine the rate at which output flows back to the client.

  While this allows a certain flexibility it can be inefficient. There will be many instances where a script will be providing just a body of data to the client, and wish to do it as quickly and efficiently as possible. Using the proposed CGI/1.2 "Script-Control:" field with the WASD extension directive X-buffer-records a script can direct the server to buffer as many script output records as possible before transfering it to the client. The following should be added to the CGI response header.

  Script-Control: X-buffer-records

  While the above offers some significant improvements to efficiency and perceived throughput the best approach is for the script to provide records the same size as the mailbox (2.2 Script Output) for detail on determining this size if required). The can be done explicitly by the script programming or if using the C language simply by changing stdout to a binary stream. With this environment the C-RTL will control output, automatically buffering as much as possible before writing it to the server.

  if ((stdout = freopen ("SYS$OUTPUT", "w", stdout, "ctx=bin")) == NULL) exit (vaxc$errno);

  Also see the section describing ‘NPH C Script’ in 2.2.2 Non-Parsed-Header Output.

C-RTL Features

Non-C Runtime Libraries (C-RTL) do not contend with records delimitted by embedded characters (the newlines and nulls, etc., of the C environment). They use VMS' and RMS' default record-oriented I/O. The C-RTL needs to accomodate the C environment's bag-o'-bytes paradigm for file content against RMS' record structures, and it's embedded terminator, stream-oriented I/O with unterminated, record-oriented I/O. Often this results in a number of issues particularly with code ported from *x environments.

The C-RTL behaviour can be modified in all sorts of ways, including some file and other I/O characteristics. The features available to such modification are incrementally increasing with each release of the C-RTL and/or C compiler. It is well advised to consult the latest release (or as appropriate for the local environment) of the Run-Time Library Reference Manual for OpenVMS Systems for the current set.

Behaviours are modified by setting various flags, either from within the program itself using thef using the decc$feature_set() and allied group of functions, or by defining an equivalent logical name, usually externally to and before executing the image. See C-RTL Reference Manual section Enabling C RTL Features Using Feature Logical Names. This is particularly useful if the source is unavailable or just as a simpler approach to modifying code.

An example of a useful feature and associated logical name is DECC$STDIO_CTX_EOL which when enabled "writing to stdout and stderr for stream access is deferred until a terminator is seen or the buffer is full" in contrast to the default behaviour of "each fwrite generates a separate write, which for mailbox and record files generates a separate record". For an application performing write()s or fwrite()s with a record-oriented <stdio> and generating inappropriate record boundaries the application could be wrapped as follows (a real-world example).

IPC Tickler

The interactions between VMS' record-oriented I/O, various run-time libraries (in particular the C-RTL), the streaming character-oriented Web, and of course WASD, can be quite complex and result in unintended output or formatting. The CGI script Inter-Process Communication (IPC) tickler WASD_ROOT:[SRC.MISC]IPCTICKLER.C is designed to allow a script programmer to gain an appreciation of how these elements interact, how WASD attempts to accomodate them, what mechanisms a script can use to explicitly convey exact requirements to WASD ... and finally, how these affect output (in particular the carriage-control) delivered to the client. If installed use /cgi-bin/IPCtickler to obtain an HTML form allowing control of several parameters into the script.

Script-Control:

The Apache Group has proposed a CGI/1.2 that includes a Script-Control: CGI response header field. WASD implements the one proposed directive, along with a number of WASD extensions (those beginning with the "X-"). Note that by convention extensions unknown by an agent should be ignored, meaning that they can be freely included, only being meaningful to WASD and not significant to other implementations.

• no-abort – The server must not terminate the script during processing for either no output or no-progress timeouts. The script is to be left completely alone to control its own termination. Caution, such scripts if problematic could easily accumulate and "clog up" a server or system.
• X-buffer-records[=0|1] – Buffer records written by the script until there is [BufferSizeDclOutput] bytes available then write it as a single block to the client. The optional zero returns the script to non-buffered records, a one enables record buffering.
• X-content-encoding-gzip[=0|1] – A script by specifying a zero can request that the server, by default encoding the response using GZIP compression, leaves the response unencoded. By specifying a one a script can request the server to encode the response using GZIP compression.
• X-content-handler=keyword – The output of the script is stripped of the CGI reponse header and the body is given to the specified content handler. Currently only the SSI engine is supported using "X-content-handler=SSI".
• X-crlf-mode[=0|1] – The server should always ensure each record has trailing carriage-return then newline characters (0x0d, 0x0a). This is generally what VMS requires for carriage control on terminals, printers, etc. The optional zero returns the script to record mode, a one enables CR-LF mode.
• X-error-line=integer – Source code module line number META in server generated error message (optional).
• X-error-module=string – Source code module name META in server generated error message (optional).
• X-error-text="string" – Text of error message (mandatory, to trigger server message generation). If without a VMS status value this is the entire error explanation. If with a VMS status it becomes the context of the status.
• X-error-vms-status=integer – VMS status value in decimal or VMS hexadecimal (e.g. %X0000002C, optional). Changes format of error message to detailed explanation of status.
• X-error-vms-text="string" – Context of a VMS status value (optional). Usually a VMS file specification of other text related to the status value (in commented content of error message).
• X-http-status=integer – Forces the HTTP response line status to this value regardless of the status in the CGI Status: response or the server status if being used as an error reporting script
• X-lifetime=value – The number of minutes before the idle scripting process is deleted by the server. Zero sets it back to the default, "none" disables this functionality.
• X-record-mode[=0|1] – The server should always ensure each record has a trailing newline character (0x0a), regardless of whether the response is a text/... content-type or not. This is what is usually required by browsers for carriage-control in text documents. The optional zero changes the script to stream mode, a one enables record mode.
• X-record0-mode[=0|1] – Some (C-RTL) carriage-control modes result in empty records being sent in lieu of carriage control (newlines). This (rather esoteric) mode only adds a newline to empty records.
• X-stream-mode[=0|1] – The server is not to adjust the carriage-control of records regardless of whether the response is a text/... content-type or not. What the script writes is exactly what the client is sent. The optional zero returns the script to record mode, a one enables stream mode.
• X-timeout-noprogress=value – The number of minutes allowed where the script does not transfer any data to the server before the server deletes the process. Zero sets it back to the default, "none" disables this functionality.
• X-timeout-output=value – The number of minutes allowed before an active script is deleted by the server, regardless of it still processing the request. Zero sets it back to the default, "none" disables this functionality.

The following is a simple example response where the server is instructed not to delete the script process under any circumstances, and that the body does not require any carriage-control changes.

Example DCL Scripts

A simple script to provide the system time might be:

A script to provide the system time more elaborately (using HTML):

2.2.2Non-Parsed-Header Output

A script does not have to output a CGI-compliant data stream. If it begins with a HTTP header status line WASD assumes it will supply a raw HTTP data stream, containing all the HTTP requirements. This is the same as or equivalent to the non-parsed-header, or "nph…" scripts of many environments. This is an example of such a script response.

Any such script must observe the HyperText Transfer Protocol, supplying a full response header and body, including correct carriage-control. Once the server detects the HTTP status header line it pays no more attention to any response header fields or body records, just transfering everything directly to the client. This can be very efficient, the server just a conduit between script and client, but does transfer the responsibility for a correct HTTP response onto the script.

NPH DCL Script

The following example shows a DCL script. Note the full HTTP header and each line explicitly terminated with a carriage-return and line-feed pair.

NPH C Script

When scripting using the C programming language there can be considerable efficiencies to be gained by providing a binary output stream from the script. This results in the C Run-Time Library (C-RTL) buffering output up to the maximum supported by the IPC mailbox. This may be enabled using a code construct similar to following to reopen stdout in binary mode.

This is used consistently in WASD scripts. Carriage-control must be supplied as part of the C standard output (no differently to any other C program). Output can be be explicitly sent to the client at any stage using the fflush() standard library function. Note that if the fwrite() function is used the current contents of the C-RTL buffer are automatically flushed along the the content of the fwrite().

2.2.3Bulk Content Output

As described above, 2.2 Script Output, the default script<->server IPC uses a mailbox. While versatile and sufficiently efficient for general use, when megabytes, tens of megabytes, and hundreds of megabytes need to be transferred, using a memory buffer shared between script and server can yield transfer improvements of up to 500%.

The script requests a memory-buffer using a CGI callout (6. CGI Callouts). Buffer size is constrained by the usual VMS 32bit memory considerations, along with available process and system resources. The server creates and maps a non-permanent global section. If this is successful the script is advised of the global section name using the callout response. The script uses this to map the section name and can then populate the buffer. When the buffer is full or otherwise ready, the script issues a callout with the number of bytes to write, and then stalls. The complete memory buffer may be written at once or any subsection of that buffer. The write is accomplished asynchronously and may comprise multiple network $QIOs or TLS/SSL blocks. When complete, a callout response to the script is issued and the script can continue processing. Standard script mailbox I/O (SYS$OUTPUT, <stdout>) and memory-buffer I/O may be interleaved as required.

The callouts are as follows:

• BUFFER-BEGIN: <integer>[k|M]

  Create a temporary global section to act as a memory buffer shared between a script process and the server. The default is Megabytes.

• BUFFER-END:

  Dispose of the shared memory buffer created by callout BUFFER-BEGIN.

• BUFFER-WRITE: <integer>

  Instruct the server to write <integer> bytes from the shared memory buffer to the client.

See working examples in WASD_ROOT:[SRC.MISC].

Actual data comparing standard mailbox IPC with memory-buffer generated using [SRC.MISC]MEMBUFDEMO.C on a HP rx2660 (1.40GHz/6.0MB) with 4 CPUs and 16383MB running VSI VMS V8.4-2L1 with Multinet UCX$IPC_SHR V55A-B147, OpenSSL 1.0.2k and WASD v11.2.0, with [BufferSizeDclOutput] 16384. In each case 250MB ("?250") is transfered via a either a 16.4kB mailbox (default) or 16.4kB memory buffer ("+b"). Significantly larger memory buffer may well improve throughput further.

It is obvious that memory-buffer provides significantly greater throughput than mailbox (from the http:// test) and that with TLS/SSL network transport the encryption becomes a significant overhead and choke-point. Nevertheless, there is still an approximate 15% dividend, plus the more efficient interface the script->memory-buffer->server provides. The VMS TLS/SSL implementation may improve with time, especially if TLS/SSL hardware engines become available with the port to x86_64.

2.3Raw HTTP Input (POST Processing)

For POST and PUT HTTP methods (e.g. a POSTed HTML form) the body of the request may be read from the HTTP$INPUT stream. For executable image scripts requiring the body to be present on SYS$INPUT (the C language stdin stream) a user-mode logical may be defined immediately before invoking the image, as in the example.

The HTTP$INPUT stream may be explicitly opened and read. Note that this is a raw stream, and HTTP lines (carriage-return/line-feed terminated sequences of characters) may have been blocked together for network transport. These would need to be explicity parsed by the program.

When scripting using the C programming language there is a tendency for the C-RTL to check for and/or add newline (0x10, <LF>) carriage-control on receipt of record (single write). While this can be useful in converting from VMS to C conventions it can also be counter-productive if the stream being received is already using C carriage-control. To prevent the C-RTL reinterpreting data passed to it it often, perhaps invariably, necessary to reopen the input stream as binary using a construct similar to following.

This, and its <stdin> equivalent (below), are used consistently in WASD scripts.

The input stream should be read before generating any output. If an error occurs during the body processing it should be reported via a CGI response header indicating an error (i.e. non-200). With HTTP/1.1 request processing there is also a requirement (that CGILIB fulfills) to return a "100 Continue" interim response after receiving the client request header and before the client sends the request body. Output of anything before this "100 Continue" is delivered will cause it to be interleaved with the script response body.

2.4CGI Function Library

A source code collection of C language functions useful for processing the more vexing aspects of CGI/CGIplus programming (1.12 Scripting Function Library).

2.5CGIUTL Utility

This assists with the generation of HTTP responses, including the transfer of binary content from files (copying a file back to the client as part of the request), and the processing of the contents of POSTed requests from DCL (2.5 CGIUTL Utility).