While commit objects usually should have only one author field, our commit parser actually handles the case where a commit has multiple author fields because some tools that exist in the wild actually write them. Detection of those additional author fields is done by using a simple `git__prefixcmp`, checking whether the current line starts with the string "author ". In case where we are handed a non-NUL-terminated string that ends directly after the space, though, we may have an out-of-bounds read of one byte when trying to compare the expected final NUL byte. Fix the issue by using `git__prefixncmp` instead of `git_prefixcmp`. Unfortunately, a test cannot be easily written to catch this case. While we could test the last error message and verify that it didn't in fact fail parsing a signature (because that would indicate that it has in fact tried to parse the additional "author " field, which it shouldn't be able to detect in the first place), this doesn't work as the next line needs to be the "committer" field, which would error out with the same error message even if we hadn't done an out-of-bounds read. As objects read from the object database are always NUL terminated, this issue cannot be triggered in normal code and thus it's not security critical.
The commit message encoding is currently being parsed by the `git__prefixcmp` function. As this function does not accept a buffer length, it will happily skip over a buffer's end if it is not `NUL` terminated. Fix the issue by using `git__prefixncmp` instead. Add a test that verifies that we are unable to parse the encoding field if it's cut off by the supplied buffer length.
Currently, parsing objects is strictly tied to having an ODB object available. This makes it hard to parse an object when all that is available is its raw object and size. Furthermore, hacking around that limitation by directly creating an ODB structure either on stack or on heap does not really work that well due to ODB objects being reference counted and then automatically free'd when reaching a reference count of zero. Implement a function `git_commit__parse_raw` to parse a commit object from a pair of `data` and `size`.
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.
Freshen the tree object that a commit points to during commit time.
When extracting a commit's signature, we first free the object and only afterwards put its signature contents into the result buffer. This works in most cases - the free'd object will normally be cached anyway, so we only end up decrementing its reference count without actually freeing its contents. But in some more exotic setups, where caching is disabled, this can definitly be a problem, as we might be the only instance currently holding a reference to this object. Fix this issue by first extracting the contents and freeing the object afterwards only.
The functions `git_commit_header_field` and `git_commit_extract_signature` both receive buffers used to hand back the results to the user. While these functions called `git_buf_sanitize` on these buffers, this is not the right thing to do, as it will simply initialize or zero-terminate passed buffers. As we want to overwrite contents, we instead have to call `git_buf_clear` to completely reset them.