In order to derive the location of some Git directories, we currently use the environment variables $HOME and $XDG_CONFIG_HOME. This might prove to be problematic whenever the binary is run with setuid, that is when the effective user does not equal the real user. In case the environment variables do not get sanitized by the caller, we thus might end up using the real user's configuration when doing stuff as the effective user. The fix is to use the passwd entry's directory instead of $HOME in this situation. As this might break scenarios where the user explicitly sets $HOME to another path, this fix is only applied in case the effective user does not equal the real user.
Next to including several files, our "common.h" header also declares various macros which are then used throughout the project. As such, we have to make sure to always include this file first in all implementation files. Otherwise, we might encounter problems or even silent behavioural differences due to macros or defines not being defined as they should be. So in fact, our header and implementation files should make sure to always include "common.h" first. This commit does so by establishing a common include pattern. Header files inside of "src" will now always include "common.h" as its first other file, separated by a newline from all the other includes to make it stand out as special. There are two cases for the implementation files. If they do have a matching header file, they will always include this one first, leading to "common.h" being transitively included as first file. If they do not have a matching header file, they instead include "common.h" as first file themselves. This fixes the outlined problems and will become our standard practice for header and source files inside of the "src/" from now on.
Provide a mechanism for callers to expand the full path of a file in the global configuration directory (that is to say, the home directory) even if the file doesn't necessarily exist. This lets callers use their own logic for building paths separate from handling file existence.
Error messages should be sentence fragments, and therefore: 1. Should not begin with a capital letter, 2. Should not conclude with punctuation, and 3. Should not end a sentence and begin a new one
We should replace it with whatever the user set, not start again.
Don't try to determine when sysdirs are uninitialized. Instead, simply initialize them all at `git_libgit2_init` time and never try to reinitialize, except when consumers explicitly call `git_sysdir_set`. Looking at the buffer length is especially problematic, since there may no appropriate path for that value. (For example, the Windows-specific programdata directory has no value on non-Windows machines.) Previously we would continually trying to re-lookup these values, which could get racy if two different threads are each calling `git_sysdir_get` and trying to lookup / clear the value simultaneously.
This is where portable git stores the global configuration which we can use to adhere to it even though git isn't quite installed on the system.
Introduce `git__getenv` which is a UTF-8 aware `getenv` everywhere. Make `cl_getenv` use this to keep consistent memory handling around return values (free everywhere, as opposed to only some platforms).