[PATCH 0/2] Diff format documentation for git-format-patch

[PATCH 0/2] Diff format documentation for git-format-patch

[Bagas Sanjaya]

git-format-patch generates diff, but the documentation for its
format is missing, so add it.

Bagas Sanjaya (2):
  git-format-patch: Include diff-generate-patch documentation
  git-format-patch: Document format for binary patch

 Documentation/git-format-patch.txt | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

-- 
2.25.1

[PATCH 1/2] git-format-patch: Include diff-generate-patch documentation

[Bagas Sanjaya]

diff-generate-patch explains format of generated patch for text files.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
---
 Documentation/git-format-patch.txt | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index 3e49bf2210..247033f8fc 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -718,6 +718,13 @@ use it only when you know the recipient uses Git to apply your patch.
 $ git format-patch -3
 ------------
 
+GENERATED DIFF FORMAT
+---------------------
+git-format-patch emits diff in generated patches. For text files, the
+diff format is described as below:
+
+include::diff-generate-patch.txt[]
+
 SEE ALSO
 --------
 linkgit:git-am[1], linkgit:git-send-email[1]
-- 
2.25.1

Re: [PATCH 1/2] git-format-patch: Include diff-generate-patch documentation

[Junio C Hamano]

Bagas Sanjaya <bagasdotme@gmail.com> writes:

> diff-generate-patch explains format of generated patch for text files.

That is true, but that does not justify this change.  In other
words, the sentence does not explain why is it a good idea to
describe the itty bitty details of what is in the format-patch
output (which is more for the consumption of somebody other than the
user who is running the format-patch command).

> Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
> ---
>  Documentation/git-format-patch.txt | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
> index 3e49bf2210..247033f8fc 100644
> --- a/Documentation/git-format-patch.txt
> +++ b/Documentation/git-format-patch.txt
> @@ -718,6 +718,13 @@ use it only when you know the recipient uses Git to apply your patch.
>  $ git format-patch -3
>  ------------
>  
> +GENERATED DIFF FORMAT
> +---------------------
> +git-format-patch emits diff in generated patches. For text files, the
> +diff format is described as below:
> +
> +include::diff-generate-patch.txt[]
> +

I suspect that, rather than adding a new section, it would be a
better change to extend what the first parapgraph of the Description
says.

The current text says something called "patch" is prepared one for
each commit, it is suitable for e-mail submission, and "am" is the
command to use it, but does not say what that "patch" really is.
The description in the page also refers to "three-dash" line, but
that is totally unclear unless the reader is given a more detailed
overview of what the "patch" the first paragraph refers to (and
diff-generate-patch.txt will not be the place to talk about it).

In other words, something like this one.

----- >8 ----- ----- >8 ----- ----- >8 ----- ----- >8 ----- ----- >8 -----
From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 24 Mar 2021 10:35:40 -0700
Subject: [PATCH] format-patch: give an overview of what a "patch" message is

The text says something called a "patch" is prepared one for each
commit, it is suitable for e-mail submission, and "am" is the
command to use it, but does not say what the "patch" really is.  The
description in the page also refers to "three-dash" line, but that
is totally unclear unless the reader is given a more detailed
overview of what the "patch" the first paragraph refers to.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/git-format-patch.txt | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index 3e49bf2210..3db0e036d8 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -36,11 +36,28 @@ SYNOPSIS
 DESCRIPTION
 -----------
 
-Prepare each commit with its patch in
-one file per commit, formatted to resemble UNIX mailbox format.
+Prepare each commit with its "patch" in
+one "message" per commit, formatted to resemble a UNIX mailbox.
 The output of this command is convenient for e-mail submission or
 for use with 'git am'.
 
+A "message" generated by the command consists of three parts:
+
+* A brief metadata header that begins with `From <commit>`
+  with a fixed `Mon Sep 17 00:00:00 2001` datestamp to help programs
+  like "file(1)" to recognize that the file is an output from this
+  command, fields that record the author identity, the author date,
+  and the title of the change (taken from the first paragraph of the
+  commit log message).
+
+* The second and subsequent paragraphs of the commit log message.
+
+* The "patch", which is the "diff -p --stat" output (see
+  linkgit:git-diff[1]) between the commit and its parent.
+
+The log message and the patch is separated by a line with a
+three-dash line.
+
 There are two ways to specify which commits to operate on.
 
 1. A single commit, <since>, specifies that the commits leading
-- 
2.31.0-464-gadc53f61d7

[PATCH 2/2] git-format-patch: Document format for binary patch

[Bagas Sanjaya]

Document binary file patch formats that are different from text file
patch.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
---
 Documentation/git-format-patch.txt | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index 247033f8fc..8de172b1f4 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -725,6 +725,28 @@ diff format is described as below:
 
 include::diff-generate-patch.txt[]
 
+Binary Files
+~~~~~~~~~~~~
+For binary files, the diff format have some differences compared to text
+files:
+
+1. Object hashes in index header line (`index <hash>..<hash> <mode>`)
+   are always given in full form, as binary patch is designed to be
+   applied only to an exact copy of original file. This is to ensure
+   that such patch don't apply to file with similar name but different
+   hash.
+
+2. There are additional extended header lines specific to binary files:
+
+        GIT binary patch
+        delta <bytes>
+        literal <bytes>
+
+3. The diff body can be either delta or full (literal) content,
+   whichever is the smallest size. It is encoded with base85 algorithm,
+   and emitted in 64 characters each line. All but the last line in
+   the body are prefixed with `z`.
+
 SEE ALSO
 --------
 linkgit:git-am[1], linkgit:git-send-email[1]
-- 
2.25.1

Re: [PATCH 2/2] git-format-patch: Document format for binary patch

[Junio C Hamano]

Bagas Sanjaya <bagasdotme@gmail.com> writes:

> Document binary file patch formats that are different from text file
> patch.
>
> Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
> ---
>  Documentation/git-format-patch.txt | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
> index 247033f8fc..8de172b1f4 100644
> --- a/Documentation/git-format-patch.txt
> +++ b/Documentation/git-format-patch.txt
> @@ -725,6 +725,28 @@ diff format is described as below:
>  
>  include::diff-generate-patch.txt[]
>  
> +Binary Files
> +~~~~~~~~~~~~
> +For binary files, the diff format have some differences compared to text
> +files:

I do not think this is specific to 'format-patch'.  If we need to
describe 'git diff --binary', it should be done there, so that
readers of "git diff --help" would also be able to learn the format.

> +1. Object hashes in index header line (`index <hash>..<hash> <mode>`)

s/Object hash/Object name/;

> +   are always given in full form, as binary patch is designed to be
> +   applied only to an exact copy of original file. This is to ensure
> +   that such patch don't apply to file with similar name but different
> +   hash.

... with similar but different object name.

cf. Documentation/glossary-contents.txt tells you what "object name" is.

> +2. There are additional extended header lines specific to binary files:
> +
> +        GIT binary patch
> +        delta <bytes>
> +        literal <bytes>
> +
> +3. The diff body can be either delta or full (literal) content,
> +   whichever is the smallest size. It is encoded with base85 algorithm,
> +   and emitted in 64 characters each line. All but the last line in
> +   the body are prefixed with `z`.

I do not think this is all that useful; it clutters the description
for a reader who is not interested in reimplementing an encoder or a
decoder from the document.

And it is way too insufficient for a reader who wants to reimplement
an encoder or a decoder.  For example,

 - It does not say anything about what the delta is and how it is
   computed.

 - The 'z' is redundant; the more important is to say that the first
   byte signals how many bytes are on that line and it is a mere
   artifact that we cram up to 52 bytes on a line.

 - It does not say anything about how the binary patch ensures that
   it is reversible (i.e. can be given to "git apply -R").

Thanks.

Re: [PATCH 2/2] git-format-patch: Document format for binary patch

[Bagas Sanjaya]

On 25/03/21 00.53, Junio C Hamano wrote:
> I do not think this is all that useful; it clutters the description
> for a reader who is not interested in reimplementing an encoder or a
> decoder from the document.
> 
> And it is way too insufficient for a reader who wants to reimplement
> an encoder or a decoder.  For example,
> 
>   - It does not say anything about what the delta is and how it is
>     computed.
> 
>   - The 'z' is redundant; the more important is to say that the first
>     byte signals how many bytes are on that line and it is a mere
>     artifact that we cram up to 52 bytes on a line.
> 
>   - It does not say anything about how the binary patch ensures that
>     it is reversible (i.e. can be given to "git apply -R").
> 
> Thanks.
> 
Hmmm...

I write this patch from "naive" observation of git format-patch's
behavior when given binary files in the commit.

Perhaps someone which is more familiar in base85 {en,de}coder and binary
patch in general can write better documentation than what I send here.

-- 
An old man doll... just what I always wanted! - Clara

Re: [PATCH 2/2] git-format-patch: Document format for binary patch

[Junio C Hamano]

Bagas Sanjaya <bagasdotme@gmail.com> writes:

> On 25/03/21 00.53, Junio C Hamano wrote:
>> I do not think this is all that useful; it clutters the description
>> for a reader who is not interested in reimplementing an encoder or a
>> decoder from the document.
>> And it is way too insufficient for a reader who wants to reimplement
>> an encoder or a decoder.  For example,
>>   - It does not say anything about what the delta is and how it is
>>     computed.
>>   - The 'z' is redundant; the more important is to say that the
>> first
>>     byte signals how many bytes are on that line and it is a mere
>>     artifact that we cram up to 52 bytes on a line.
>>   - It does not say anything about how the binary patch ensures that
>>     it is reversible (i.e. can be given to "git apply -R").
>> Thanks.
>> 
> Hmmm...
>
> I write this patch from "naive" observation of git format-patch's
> behavior when given binary files in the commit.
>
> Perhaps someone which is more familiar in base85 {en,de}coder and binary
> patch in general can write better documentation than what I send here.

I do not mind reviewing an update to an existing document or a new
document in Documentation/technical/ somewhere, if somebody is
motivated enough to write the details to a degree that would allow
reimplementation of the encoder and the decoder.  I just do not think
it belongs to the end-user-facing document of "format-patch", whose
target is users of the "format-patch" command, not reimplementors of
the command.