[PATCH v2 0/2] mingw: handle absolute paths in expand_user_path()

In Git for Windows, we ran with a patch "in production" for quite a while where paths starting with a slash (i.e. looking like Unix paths, not like Windows paths) were interpreted as being relative to the runtime prefix, when expanded via expand_user_path().

This was sent to the Git mailing list as a discussion starter, and it was pointed out that this is neither portable nor unambiguous.

After the dust settled, I thought about the presented ideas for a while (quite a while...), and ended up with the following: any path starting with <RUNTIME-PREFIX>/ is expanded. This is ambiguous because it could be a valid path. But then, it is unlikely, and if someone really wants to specify such a path, it is easy to slap a ./ in front and they're done.

Changes since v1:

 * Included a test for the RUNTIME_PREFIX that I had meant to send for ages
   already, and based on which...
 * A test case was added to verify that this actually works as intended
 * It is no longer Windows-specific
 * I added some documentation

Johannes Schindelin (2):
  tests: exercise the RUNTIME_PREFIX feature
  expand_user_path(): support specifying paths relative to the runtime
    prefix

 Documentation/config.txt | 10 ++++++++++
 Makefile                 |  5 +++++
 path.c                   |  5 +++++
 t/t0060-path-utils.sh    | 26 ++++++++++++++++++++++++++
 4 files changed, 46 insertions(+)

base-commit: ebf3c04b262aa27fbb97f8a0156c2347fecafafb
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-66%2Fdscho%2Fmingw-expand-absolute-user-path-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-66/dscho/mingw-expand-absolute-user-path-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/66

Range-diff vs v1:

 -:  ----------- > 1:  cc8f09baba9 tests: exercise the RUNTIME_PREFIX feature
 1:  2287dd96cf0 ! 2:  66df56f5db0 mingw: handle absolute paths in expand_user_path()
     @@ Metadata
      Author: Johannes Schindelin <Johannes.Schindelin@gmx.de>
      
       ## Commit message ##
     -    mingw: handle absolute paths in expand_user_path()
     +    expand_user_path(): support specifying paths relative to the runtime prefix
      
     -    On Windows, an absolute POSIX path needs to be turned into a Windows
     -    one.
     +    Ever since Git learned to detect its install location at runtime, there
     +    was the slightly awkward problem that it was impossible to specify paths
     +    relative to said location.
     +
     +    For example, if a version of Git was shipped with custom SSL
     +    certificates to use, there was no portable way to specify
     +    `http.sslCAInfo`.
     +
     +    In Git for Windows, the problem was "solved" for years by interpreting
     +    paths starting with a slash as relative to the runtime prefix.
     +
     +    However, this is not correct: such paths _are_ legal on Windows, and
     +    they are interpreted as absolute paths in the same drive as the current
     +    directory.
     +
     +    After a lengthy discussion, and a way lengthier time to mull over the
     +    problem and its best solution, we decided to introduce support for the
     +    magic sequence `<RUNTIME-PREFIX>/`. If a path starts with this, the
     +    remainder is interpreted as relative to the detected runtime prefix.
     +
     +    This solves the problem, but what new problems does it stir up? Here are
     +    the two most obvious ones:
     +
     +    - What if Git was not compiled with support for a runtime prefix?
     +
     +      In that case, we will simply use the compiled-in hard-coded prefix.
     +
     +    - What if a user _wants_ to specify a path starting with the magic
     +      sequence?
     +
     +      In that case, the user will simply need to prefix the magic sequence
     +      with `./` and voilà, the path won't be expanded.
      
          Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
      
     + ## Documentation/config.txt ##
     +@@ Documentation/config.txt: pathname::
     + 	tilde expansion happens to such a string: `~/`
     + 	is expanded to the value of `$HOME`, and `~user/` to the
     + 	specified user's home directory.
     +++
     ++If a path starts with `<RUNTIME-PREFIX>/`, the remainder is
     ++interpreted as a path relative to Git's "runtime prefix", i.e. relative
     ++to the location where Git itself was installed. For example,
     ++`<RUNTIME-PREFIX>/bin/` refers to the directory in which the Git
     ++executable itself lives. If Git was compiled without runtime prefix
     ++support, the compiled-in prefix will be subsituted instead. In the
     ++unlikely event that a literal path needs to be specified that should
     ++_not_ be expanded, it needs to be prefixed by `./`, like so:
     ++`./<RUNTIME-PREFIX>/bin`.
     + 
     + 
     + Variables
     +
       ## path.c ##
      @@
     - #include "path.h"
       #include "packfile.h"
       #include "object-store.h"
     + #include "lockfile.h"
      +#include "exec-cmd.h"
       
       static int get_st_mode_bits(const char *path, int *mode)
     @@ path.c: char *expand_user_path(const char *path, int real_home)
       
       	if (path == NULL)
       		goto return_null;
     -+#ifdef __MINGW32__
     -+	if (path[0] == '/')
     -+		return system_path(path + 1);
     -+#endif
     ++
     ++	if (skip_prefix(path, "<RUNTIME-PREFIX>/", &path))
     ++		return system_path(path);
     ++
       	if (path[0] == '~') {
       		const char *first_slash = strchrnul(path, '/');
       		const char *username = path + 1;
     +
     + ## t/t0060-path-utils.sh ##
     +@@ t/t0060-path-utils.sh: test_expect_success RUNTIME_PREFIX,CAN_EXEC_IN_PWD 'RUNTIME_PREFIX works' '
     + 	cp "$GIT_EXEC_PATH"/git$X pretend/bin/ &&
     + 	GIT_EXEC_PATH= ./pretend/bin/git here >actual &&
     + 	echo HERE >expect &&
     ++	test_cmp expect actual'
     ++
     ++test_expect_success RUNTIME_PREFIX,CAN_EXEC_IN_PWD '<RUNTIME-PREFIX>/ works' '
     ++	mkdir -p pretend/bin &&
     ++	cp "$GIT_EXEC_PATH"/git$X pretend/bin/ &&
     ++	git config yes.path "<RUNTIME-PREFIX>/yes" &&
     ++	GIT_EXEC_PATH= ./pretend/bin/git config --path yes.path >actual &&
     ++	echo "$(pwd)/pretend/yes" >expect &&
     + 	test_cmp expect actual
     + '
     +