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ABNF Notation

ABNF notation as described by RFC 5234 is used within the protocol documents, except the following replacement core rules are used:

  HEXDIG    =  DIGIT / "a" / "b" / "c" / "d" / "e" / "f"

We also define the following common rules:

  NUL       =  %x00
  zero-id   =  40*"0"
  obj-id    =  40*(HEXDIGIT)

  refname  =  "HEAD"
  refname /=  "refs/" <see discussion below>

A refname is a hierarchical octet string beginning with "refs/" and not violating the git-check-ref-format command’s validation rules. More specifically, they:

  1. They can include slash / for hierarchical (directory) grouping, but no slash-separated component can begin with a dot ..

  2. They must contain at least one /. This enforces the presence of a category like heads/, tags/ etc. but the actual names are not restricted.

  3. They cannot have two consecutive dots .. anywhere.

  4. They cannot have ASCII control characters (i.e. bytes whose values are lower than \040, or \177 DEL), space, tilde ~, caret ^, colon :, question-mark ?, asterisk *, or open bracket [ anywhere.

  5. They cannot end with a slash / or a dot ..

  6. They cannot end with the sequence .lock.

  7. They cannot contain a sequence @{.

  8. They cannot contain a \\.

pkt-line Format

Much (but not all) of the payload is described around pkt-lines.

A pkt-line is a variable length binary string. The first four bytes of the line, the pkt-len, indicates the total length of the line, in hexadecimal. The pkt-len includes the 4 bytes used to contain the length’s hexadecimal representation.

A pkt-line MAY contain binary data, so implementors MUST ensure pkt-line parsing/formatting routines are 8-bit clean.

A non-binary line SHOULD BE terminated by an LF, which if present MUST be included in the total length.

The maximum length of a pkt-line’s data component is 65520 bytes. Implementations MUST NOT send pkt-line whose length exceeds 65524 (65520 bytes of payload + 4 bytes of length data).

Implementations SHOULD NOT send an empty pkt-line ("0004").

A pkt-line with a length field of 0 ("0000"), called a flush-pkt, is a special case and MUST be handled differently than an empty pkt-line ("0004").

  pkt-line     =  data-pkt / flush-pkt

  data-pkt     =  pkt-len pkt-payload
  pkt-len      =  4*(HEXDIG)
  pkt-payload  =  (pkt-len - 4)*(OCTET)

  flush-pkt    = "0000"

Examples (as C-style strings):

  pkt-line          actual value
  ---------------------------------
  "0006a\n"         "a\n"
  "0005a"           "a"
  "000bfoobar\n"    "foobar\n"
  "0004"            ""
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