Before printing into a `git_buf` structure, we always call `ENSURE_SIZE` first. This macro will reallocate the buffer as-needed depending on whether the current amount of allocated bytes is sufficient or not. If `asize` is big enough, then it will just do nothing, otherwise it will call out to `git_buf_try_grow`. But in fact, it is insufficient to only check `asize`. When we fail to allocate any more bytes e.g. via `git_buf_try_grow`, then we set the buffer's pointer to `git_buf__oom`. Note that we touch neither `asize` nor `size`. So if we just check `asize > targetsize`, then we will happily let the caller of `ENSURE_SIZE` proceed with an out-of-memory buffer. As a result, we will print all bytes into the out-of-memory buffer instead, resulting in an out-of-bounds write. Fix the issue by having `ENSURE_SIZE` verify that the buffer is not marked as OOM. Add a test to verify that we're not writing into the OOM buffer.
When growing buffers, we repeatedly multiply the currently allocated
number of bytes by 1.5 until it exceeds the requested number of bytes.
This has two major problems:
1. If the current number of bytes is tiny and one wishes to resize
to a comparatively huge number of bytes, then we may need to loop
thousands of times.
2. If resizing to a value close to `SIZE_MAX` (which would fail
anyway), then we probably hit an infinite loop as multiplying the
current amount of bytes will repeatedly result in integer
overflows.
When reallocating buffers, one typically chooses values close to 1.5 to
enable re-use of resulting memory holes in later reallocations. But
because of this, it really only makes sense to use a factor of 1.5
_once_, but not looping until we finally are able to fit it. Thus, we
can completely avoid the loop and just opt for the much simpler
algorithm of multiplying with 1.5 once and, if the result doesn't fit,
just use the target size. This avoids both problems of looping
extensively and hitting overflows.
This commit also adds a test that would've previously resulted in an
infinite loop.
If growing a buffer fails, we set its pointer to the static `git_buf__oom` structure. While we correctly free the old pointer if `git__malloc` returned an error, we do not free it if there was an integer overflow while calculating the new allocation size. Fix this issue by freeing the pointer to plug the memory leak.
Quiet down a warning from MSVC about how we're potentially losing data. This is safe since we've explicitly tested it.
Move to the `git_error` name in the internal API for error-related functions.
Introduce a function to take a percent-encoded string (URI encoded, described by RFC 1738) and decode it into a `git_buf`.
Both the `git_buf_init` and `git_buf_attach` functions may call `git_buf_grow` in case they were given an allocation length as parameter. As such, it is possible for these functions to fail when we run out of memory. While it won't probably be used anytime soon, it does indeed make sense to also record this fact by returning an error code from both functions. As they belong to the internal API only, this change does not break our interface.
The `ENSURE_SIZE` macro can be used to grow a buffer if its currently allocated size does not suffice a required target size. While most of the code already uses this macro, the `git_buf_join` and `git_buf_join3` functions do not yet use it. Due to the macro first checking whether we have to grow the buffer at all, this has the benefit of saving a function call when it is not needed. While this is nice to have, it will probably not matter at all performance-wise -- instead, this only serves for consistency across the code.