[PATCH v3] credential: treat CR/LF as line endings in the credential protocol

From: Nikita Leonov <nykyta.leonov@gmail.com>

This fix makes using Git credentials more friendly to Windows users: it allows a credential helper to communicate using CR/LF line endings ("DOS line endings" commonly found on Windows) instead of LF-only line endings ("Unix line endings").

Note that this changes the behavior a bit: if a credential helper produces, say, a password with a trailing Carriage Return character, that will now be culled even when the rest of the lines end only in Line Feed characters, indicating that the Carriage Return was not meant to be part of the line ending.

In practice, it seems _very_ unlikely that something like this happens. Passwords usually need to consist of non-control characters, URLs need to have special characters URL-encoded, and user names, well, are names.

However, it _does_ help on Windows, where CR/LF line endings are common: as unrecognized commands are simply ignored by the credential machinery, even a command like `quit\r` (which is clearly intended to abort) would simply be ignored (silently) by Git.

So let's change the credential machinery to accept both CR/LF and LF line endings.

While we do this for the credential helper protocol, we do _not_ adjust `git credential-cache--daemon` (which won't work on Windows, anyway, because it requires Unix sockets) nor `git credential-store` (which writes the file `~/.git-credentials` which we consider an implementation detail that should be opaque to the user, read: we do expect users _not_ to edit this file manually).

Signed-off-by: Nikita Leonov <nykyta.leonov@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    Prepare git credential to read input with DOS line endings
    
    This contribution came in via Git for Windows
    [https://github.com/git-for-windows/git/pull/2516].
    
    Sadly, I did not find the time to go through all the changes of 8f309aeb
    ("strbuf: introduce strbuf_getline_{lf,nul}()", 2016-01-13) (as Junio
    asked
    [https://public-inbox.org/git/xmqqmu9lnjdh.fsf@gitster-ct.c.googlers.com]
    ). Rather than delaying this patch indefinitely, I admit defeat on that
    angle.
    
    Changes since v2:
    
     * Dropped the credential-cache--daemon and credential-store changes
       again.
     * Enhanced the commit message (also explaining why we don't touch the
       daemon and the store).
    
    Changes since v1:
    
     * Added a commit to adjust credential-daemon and credential-store in
       the same manner.
     * Adjusted the documentation accordingly.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-710%2Fdscho%2Fcrlf-aware-git-credential-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-710/dscho/crlf-aware-git-credential-v3
Pull-Request: https://github.com/git/git/pull/710

Range-diff vs v2:

 1:  27f6400a21 ! 1:  f6eeb18d3a credential.c: fix credential reading with regards to CR/LF
     @@ Metadata
      Author: Nikita Leonov <nykyta.leonov@gmail.com>
      
       ## Commit message ##
     -    credential.c: fix credential reading with regards to CR/LF
     +    credential: treat CR/LF as line endings in the credential protocol
      
     -    This fix makes using Git credentials more friendly to Windows users. In
     -    previous version it was unable to finish input correctly without
     -    configuration changes (tested in PowerShell, CMD, Cygwin).
     +    This fix makes using Git credentials more friendly to Windows users: it
     +    allows a credential helper to communicate using CR/LF line endings ("DOS
     +    line endings" commonly found on Windows) instead of LF-only line endings
     +    ("Unix line endings").
      
     -    We know credential filling should be finished by empty input, but the
     -    current implementation does not take into account CR/LF ending, and
     -    hence instead of the empty string we get '\r', which is interpreted as
     -    an incorrect string.
     +    Note that this changes the behavior a bit: if a credential helper
     +    produces, say, a password with a trailing Carriage Return character,
     +    that will now be culled even when the rest of the lines end only in Line
     +    Feed characters, indicating that the Carriage Return was not meant to be
     +    part of the line ending.
      
     -    So this commit changes default reading function to a more Windows
     -    compatible reading function.
     +    In practice, it seems _very_ unlikely that something like this happens.
     +    Passwords usually need to consist of non-control characters, URLs need
     +    to have special characters URL-encoded, and user names, well, are names.
     +
     +    However, it _does_ help on Windows, where CR/LF line endings are common:
     +    as unrecognized commands are simply ignored by the credential machinery,
     +    even a command like `quit\r` (which is clearly intended to abort) would
     +    simply be ignored (silently) by Git.
     +
     +    So let's change the credential machinery to accept both CR/LF and LF
     +    line endings.
     +
     +    While we do this for the credential helper protocol, we do _not_ adjust
     +    `git credential-cache--daemon` (which won't work on Windows, anyway,
     +    because it requires Unix sockets) nor `git credential-store` (which
     +    writes the file `~/.git-credentials` which we consider an implementation
     +    detail that should be opaque to the user, read: we do expect users _not_
     +    to edit this file manually).
      
          Signed-off-by: Nikita Leonov <nykyta.leonov@gmail.com>
          Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
 2:  f69076036f < -:  ---------- credentials: make line reading Windows compatible
 3:  61baea1061 < -:  ---------- docs: make notes regarding credential line reading

 credential.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)