Only avoid `mmap(2)`/`ftruncate(2)` when in non-Windows

It turns out that if we use `mmap(2)`, non-Windows remote filesystems
break due to permissions. If we don't, _Windows_ remote filesystems
break due to lack of coherence between memory mapped views of the file
and direct I/O operations done to the files.

To break out of this impossible situation, conditionally-compile
versions of Windows-specific `write_at` and `append_to_pack`.

src/indexer.c

@@ -601,6 +601,8 @@ static void hash_partially(git_indexer *idx, const uint8_t *data, size_t size)
 	idx->inbuf_len += size - to_expell;
 }
 
+#if defined(NO_MMAP) || !defined(GIT_WIN32)
+
 static int write_at(git_indexer *idx, const void *data, off64_t offset, size_t size)
 {
 	size_t remaining_size = size;
@@ -624,17 +626,86 @@ static int write_at(git_indexer *idx, const void *data, off64_t offset, size_t s
 
 static int append_to_pack(git_indexer *idx, const void *data, size_t size)
 {
+	if (write_at(idx, data, idx->pack->mwf.size, size) < 0) {
+		git_error_set(GIT_ERROR_OS, "cannot extend packfile '%s'", idx->pack->pack_name);
+		return -1;
+	}
+
+	return 0;
+}
+
+#else
+
+/*
+ * Windows may keep different views to a networked file for the mmap- and
+ * open-accessed versions of a file, so any writes done through
+ * `write(2)`/`pwrite(2)` may not be reflected on the data that `mmap(2)` is
+ * able to read.
+ */
+
+static int write_at(git_indexer *idx, const void *data, off64_t offset, size_t size)
+{
+	git_file fd = idx->pack->mwf.fd;
+	size_t mmap_alignment;
+	size_t page_offset;
+	off64_t page_start;
+	unsigned char *map_data;
+	git_map map;
+	int error;
+
+	GIT_ASSERT_ARG(data);
+	GIT_ASSERT_ARG(size);
+
+	if ((error = git__mmap_alignment(&mmap_alignment)) < 0)
+		return error;
+
+	/* the offset needs to be at the mmap boundary for the platform */
+	page_offset = offset % mmap_alignment;
+	page_start = offset - page_offset;
+
+	if ((error = p_mmap(&map, page_offset + size, GIT_PROT_WRITE, GIT_MAP_SHARED, fd, page_start)) < 0)
+		return error;
+
+	map_data = (unsigned char *)map.data;
+	memcpy(map_data + page_offset, data, size);
+	p_munmap(&map);
+
+	return 0;
+}
+
+static int append_to_pack(git_indexer *idx, const void *data, size_t size)
+{
+	off64_t new_size;
+	size_t mmap_alignment;
+	size_t page_offset;
+	off64_t page_start;
+	off64_t current_size = idx->pack->mwf.size;
+	int error;
+
 	if (!size)
 		return 0;
 
-	if (write_at(idx, data, idx->pack->mwf.size, size) < 0) {
+	if ((error = git__mmap_alignment(&mmap_alignment)) < 0)
+		return error;
+
+	/* Write a single byte to force the file system to allocate space now or
+	 * report an error, since we can't report errors when writing using mmap.
+	 * Round the size up to the nearest page so that we only need to perform file
+	 * I/O when we add a page, instead of whenever we write even a single byte. */
+	new_size = current_size + size;
+	page_offset = new_size % mmap_alignment;
+	page_start = new_size - page_offset;
+
+	if (p_pwrite(idx->pack->mwf.fd, data, 1, page_start + mmap_alignment - 1) < 0) {
 		git_error_set(GIT_ERROR_OS, "cannot extend packfile '%s'", idx->pack->pack_name);
 		return -1;
 	}
 
-	return 0;
+	return write_at(idx, data, idx->pack->mwf.size, size);
 }
 
+#endif
+
 static int read_stream_object(git_indexer *idx, git_indexer_progress *stats)
 {
 	git_packfile_stream *stream = &idx->stream;
@@ -1234,6 +1305,17 @@ int git_indexer_commit(git_indexer *idx, git_indexer_progress *stats)
 	if (git_mwindow_free_all(&idx->pack->mwf) < 0)
 		goto on_error;
 
+#if !defined(NO_MMAP) && defined(GIT_WIN32)
+	/*
+	 * Truncate file to undo rounding up to next page_size in append_to_pack only
+	 * when mmap was used, to prevent failures in non-Windows remote filesystems.
+	 */
+	if (p_ftruncate(idx->pack->mwf.fd, idx->pack->mwf.size) < 0) {
+		git_error_set(GIT_ERROR_OS, "failed to truncate pack file '%s'", idx->pack->pack_name);
+		return -1;
+	}
+#endif
+
 	if (idx->do_fsync && p_fsync(idx->pack->mwf.fd) < 0) {
 		git_error_set(GIT_ERROR_OS, "failed to fsync packfile");
 		goto on_error;