VMOD blob - Utilities for the VCL blob type, encoding and decoding

SYNOPSIS

import blob [from "path"]

BLOB decode(ENUM decoding, INT length, STRING encoded)

STRING encode(ENUM encoding, ENUM case, BLOB blob)

STRING transcode(ENUM decoding, ENUM encoding, ENUM case, INT length, STRING encoded)

BOOL same(BLOB, BLOB)

BOOL equal(BLOB, BLOB)

INT length(BLOB)

BLOB sub(BLOB, BYTES length, BYTES offset=0)

new xblob = blob.blob(ENUM decoding, STRING encoded)

    BLOB xblob.get()

    STRING xblob.encode(ENUM encoding, ENUM case)

DESCRIPTION

This VMOD provides utility functions and an object for the VCL data type BLOB, which may contain arbitrary data of any length.

Examples:

sub vcl_init {
    # Create blob objects from encodings such as base64 or hex.
    new myblob   = blob.blob(BASE64, "Zm9vYmFy");
    new yourblob = blob.blob(encoded="666F6F", decoding=HEX);
}

sub vcl_deliver {
    # The .get() method retrieves the BLOB from an object.
    set resp.http.MyBlob-As-Hex
        = blob.encode(blob=myblob.get(), encoding=HEX);

    # The .encode() method efficiently retrieves an encoding.
    set resp.http.YourBlob-As-Base64 = yourblob.encode(BASE64);

    # decode() and encode() functions convert blobs to text and
    # vice versa at runtime.
    set resp.http.Base64-Encoded
        = blob.encode(BASE64,
                      blob=blob.decode(HEX,
                                       encoded=req.http.Hex-Encoded));
}

sub vcl_recv {
    # transcode() converts from one encoding to another.
    # case=UPPER specifies upper-case hex digits A-F.
    set req.http.Hex-Encoded
        = blob.transcode(decoding=BASE64, encoding=HEX,
                         case=UPPER, encoded="YmF6");

    # transcode() from URL to IDENTITY effects a URL decode.
    set req.url = blob.transcode(encoded=req.url, decoding=URL);

    # transcode() from IDENTITY to URL effects a URL encode.
    set req.http.url_urlcoded
        = blob.transcode(encoded=req.url, encoding=URL);
}

ENCODING SCHEMES

Binary-to-text encoding schemes are specified by ENUMs in the VMOD’s constructor, methods and functions. Decodings convert a (possibly concatenated) string into a blob, while encodings convert a blob into a string.

ENUM values for an encoding scheme can be one of:

  • IDENTITY

  • BASE64

  • BASE64URL

  • BASE64URLNOPAD

  • HEX

  • URL

Empty strings are decoded into a “null blob” (of length 0), and conversely a null blob is encoded as the empty string.

For encodings with HEX or URL, you may also specify a case ENUM with one of the values LOWER, UPPER or DEFAULT to produce a string with lower- or uppercase hex digits (in [a-f] or [A-F]). The default value for case is DEFAULT, which for HEX and URL means the same as LOWER.

The case ENUM is not relevant for decodings; HEX or URL strings to be decoded as BLOBs may have hex digits in either case, or in mixed case.

The case ENUM MUST be set to DEFAULT for the other encodings (BASE64* and IDENTITY). You cannot, for example, produce an uppercase string by using the IDENTITY scheme with case=UPPER. To change the case of a string, use the std.toupper() or std.tolower() functions from VMOD std - Varnish Standard Module.

IDENTITY

The simplest encoding converts between the BLOB and STRING data types, leaving the contents byte-identical.

Note that a BLOB may contain a null byte at any position before its end; if such a BLOB is decoded with IDENTITY, the resulting STRING will have a null byte at that position. Since VCL strings, like C strings, are represented with a terminating null byte, the string will be truncated, appearing to contain less data than the original blob. For example:

# Decode from the hex encoding for "foo\0bar".
# The header will be seen as "foo".
set resp.http.Trunced-Foo1
    = blob.encode(IDENTITY, blob=blob.decode(HEX,
                                             encoded="666f6f00626172"));

IDENTITY is the default encoding and decoding. So the above can also be written as:

# Decode from the hex encoding for "foo\0bar".
# The header will be seen as "foo".
set resp.http.Trunced-Foo2
  = blob.encode(blob=blob.decode(HEX, encoded="666f6f00626172"));

The case ENUM MUST be set to DEFAULT for IDENTITY encodings.

BASE64*

The base64 encoding schemes use 4 characters to encode 3 bytes. There are no newlines or maximal line lengths – whitespace is not permitted.

The BASE64 encoding uses the alphanumeric characters, + and /; and encoded strings are padded with the = character so that their length is always a multiple of four.

The BASE64URL encoding also uses the alphanumeric characters, but - and _ instead of + and /, so that an encoded string can be used safely in a URL. This scheme also uses the padding character =.

The BASE64URLNOPAD encoding uses the same alphabet as BASE6URL, but leaves out the padding. Thus the length of an encoding with this scheme is not necessarily a multiple of four.

The case ENUM MUST be set to DEFAULT for for all of the BASE64* encodings.

HEX

The HEX encoding scheme converts hex strings into blobs and vice versa. For encodings, you may use the case ENUM to specify upper- or lowercase hex digits A through f (default DEFAULT, which means the same as LOWER). A prefix such as 0x is not used for an encoding and is illegal for a decoding.

If a hex string to be decoded has an odd number of digits, it is decoded as if a 0 is prepended to it; that is, the first digit is interpreted as representing the least significant nibble of the first byte. For example:

# The concatenated string is "abcdef0", and is decoded as "0abcdef0".
set resp.http.First = "abc";
set resp.http.Second = "def0";
set resp.http.Hex-Decoded
    = blob.encode(HEX, blob=blob.decode(HEX,
                       encoded=resp.http.First + resp.http.Second));

URL

The URL decoding replaces any %<2-hex-digits> substrings with the binary value of the hexadecimal number after the % sign.

The URL encoding implements “percent encoding” as per RFC3986. The case ENUM determines the case of the hex digits, but does not affect alphabetic characters that are not percent-encoded.

BLOB decode(ENUM decoding, INT length, STRING encoded)

BLOB decode(
   ENUM {IDENTITY, BASE64, BASE64URL, BASE64URLNOPAD, HEX, URL} decoding=IDENTITY,
   INT length=0,
   STRING encoded
)

Returns the BLOB derived from the string encoded according to the scheme specified by decoding.

If length > 0, only decode the first length characters of the encoded string. If length <= 0 or greater than the length of the string, then decode the entire string. The default value of length is 0.

decoding defaults to IDENTITY.

Example:

blob.decode(BASE64, encoded="Zm9vYmFyYmF6");

# same with named parameters
blob.decode(encoded="Zm9vYmFyYmF6", decoding=BASE64);

# convert string to blob
blob.decode(encoded="foo");

STRING encode(ENUM encoding, ENUM case, BLOB blob)

STRING encode(
   ENUM {IDENTITY, BASE64, BASE64URL, BASE64URLNOPAD, HEX, URL} encoding=IDENTITY,
   ENUM {LOWER, UPPER, DEFAULT} case=DEFAULT,
   BLOB blob
)

Returns a string representation of the BLOB blob as specified by encoding. case determines the case of hex digits for the HEX and URL encodings, and is ignored for the other encodings.

encoding defaults to IDENTITY, and case defaults to DEFAULT. DEFAULT is interpreted as LOWER for the HEX and URL encodings, and is the required value for the other encodings.

Example:

set resp.http.encode1
    = blob.encode(HEX,
                  blob=blob.decode(BASE64, encoded="Zm9vYmFyYmF6"));

# same with named parameters
set resp.http.encode2
    = blob.encode(blob=blob.decode(encoded="Zm9vYmFyYmF6",
                                           decoding=BASE64),
                      encoding=HEX);

# convert blob to string
set resp.http.encode3
    = blob.encode(blob=blob.decode(encoded="foo"));

STRING transcode(ENUM decoding, ENUM encoding, ENUM case, INT length, STRING encoded)

STRING transcode(
   ENUM {IDENTITY, BASE64, BASE64URL, BASE64URLNOPAD, HEX, URL} decoding=IDENTITY,
   ENUM {IDENTITY, BASE64, BASE64URL, BASE64URLNOPAD, HEX, URL} encoding=IDENTITY,
   ENUM {LOWER, UPPER, DEFAULT} case=DEFAULT,
   INT length=0,
   STRING encoded
)

Translates from one encoding to another, by first decoding the string encoded according to the scheme decoding, and then returning the encoding of the resulting blob according to the scheme encoding. case determines the case of hex digits for the HEX and URL encodings, and is ignored for other encodings.

As with vmod_blob.decode: If length > 0, only decode the first length characters of the encoded string, otherwise decode the entire string. The default value of length is 0.

decoding and encoding default to IDENTITY, and case defaults to DEFAULT. DEFAULT is interpreted as LOWER for the HEX and URL encodings, and is the required value for the other encodings.

Example:

set resp.http.Hex2Base64-1
     = blob.transcode(HEX, BASE64, encoded="666f6f");

 # same with named parameters
 set resp.http.Hex2Base64-2
    = blob.transcode(encoded="666f6f",
                          encoding=BASE64, decoding=HEX);

 # URL decode -- recall that IDENTITY is the default encoding.
 set resp.http.urldecoded
    = blob.transcode(encoded="foo%20bar", decoding=URL);

 # URL encode
 set resp.http.urlencoded
     = blob.transcode(encoded="foo bar", encoding=URL);

BOOL same(BLOB, BLOB)

Returns true if and only if the two BLOB arguments are the same object, i.e. they specify exactly the same region of memory, or both are empty.

If the BLOBs are both empty (length is 0 and/or the internal pointer is NULL), then vmod_blob.same returns true. If any non-empty BLOB is compared to an empty BLOB, then vmod_blob.same returns false.

BOOL equal(BLOB, BLOB)

Returns true if and only if the two BLOB arguments have equal contents (possibly in different memory regions).

As with vmod_blob.same: If the BLOBs are both empty, then vmod_blob.equal returns true. If any non-empty BLOB is compared to an empty BLOB, then vmod_blob.equal returns false.

INT length(BLOB)

Returns the length of the BLOB.

BLOB sub(BLOB, BYTES length, BYTES offset=0)

Returns a new BLOB formed from length bytes of the BLOB argument starting at offset bytes from the start of its memory region. The default value of offset is 0B.

vmod_blob.sub fails and returns NULL if the BLOB argument is empty, or if offset + length requires more bytes than are available in the BLOB.

new xblob = blob.blob(ENUM decoding, STRING encoded)

new xblob = blob.blob(
   ENUM {IDENTITY, BASE64, BASE64URL, BASE64URLNOPAD, HEX, URL} decoding=IDENTITY,
   STRING encoded
)

Creates an object that contains the BLOB derived from the string encoded according to the scheme decoding.

Example:

new theblob1 = blob.blob(BASE64, encoded="YmxvYg==");

# same with named arguments
new theblob2 = blob.blob(encoded="YmxvYg==", decoding=BASE64);

# string as a blob
new stringblob = blob.blob(encoded="bazz");

BLOB xblob.get()

Returns the BLOB created by the constructor.

Example:

set resp.http.The-Blob1 =
    blob.encode(blob=theblob1.get());

set resp.http.The-Blob2 =
    blob.encode(blob=theblob2.get());

set resp.http.The-Stringblob =
    blob.encode(blob=stringblob.get());

STRING xblob.encode(ENUM encoding, ENUM case)

STRING xblob.encode(
      ENUM {IDENTITY, BASE64, BASE64URL, BASE64URLNOPAD, HEX, URL} encoding=IDENTITY,
      ENUM {LOWER, UPPER, DEFAULT} case=DEFAULT
)

Returns an encoding of BLOB created by the constructor, according to the scheme encoding. case determines the case of hex digits for the HEX and URL encodings, and MUST be set to DEFAULT for the other encodings.

Example:

# blob as text
set resp.http.The-Blob = theblob1.encode();

# blob as base64
set resp.http.The-Blob-b64 = theblob1.encode(BASE64);

For any vmod_blob.blob object, encoding and case, encodings via the vmod_blob.blob.encode method and the vmod_blob.encode function are equal:

# Always true:
blob.encode(ENC, CASE, blob.get()) == blob.encode(ENC, CASE)

But the vmod_blob.blob.encode object method is more efficient – the encoding is computed once and cached (with allocation in heap memory), and the cached encoding is retrieved on every subsequent call. The vmod_blob.encode function computes the encoding on every call, allocating space for the string in Varnish workspaces.

So if the data in a BLOB are fixed at VCL initialization time, so that its encodings will always be the same, it is better to create a vmod_blob.blob object. The VMOD’s functions should be used for data that are not known until runtime.

ERRORS

The encoders, decoders and vmod_blob.sub may fail if there is insufficient space to create the new blob or string. Decoders may also fail if the encoded string is an illegal format for the decoding scheme. Encoders will fail for the IDENTITY and BASE64* encoding schemes if the case ENUM is not set to DEFAULT.

If any of the VMOD’s methods, functions or constructor fail, then VCL failure is invoked, just as if return(fail) had been called in the VCL source. This means that:

  • If the vmod_blob.blob`_ object constructor fails, or if any methods or functions fail during ``vcl_init{}, then the VCL program will fail to load, and the VCC compiler will emit an error message.

  • If a method or function fails in any other VCL subroutine besides vcl_synth{}, then control is directed to vcl_synth{}. The response status is set to 503 with the reason string "VCL failed", and an error message will be written to the VSL using the tag VCL_Error.

  • If the failure occurs during vcl_synth{}, then vcl_synth{} is aborted. The response line "503 VCL failed" is returned, and the VCL_Error message is written to the log.

LIMITATIONS

The VMOD allocates memory in various ways for new blobs and strings. The vmod_blob.blob object and its methods allocate memory from the heap, and hence they are only limited by available virtual memory.

The vmod_blob.encode, vmod_blob.decode and vmod_blob.transcode functions allocate Varnish workspace, as does vmod_blob.sub for the newly created BLOB. If these functions are failing, as indicated by “out of space” messages in the Varnish log (with the VCL_Error tag), then you will need to increase the varnishd parameters workspace_client and/or workspace_backend.

The vmod_blob.transcode function also allocates space on the stack for a temporary BLOB. If this function causes stack overflow, you may need to increase the varnishd parameter thread_pool_stack.

SEE ALSO