keymap: add xkb_keymap_new_from_buffer()

The current API doesn't allow the caller to create keymaps from mmap()'ed
files. The problem is, xkb_keymap_new_from_string() requires a terminating
0 byte. However, there is no way to guarantee that when using mmap() so a
user currently has to copy the whole file just to get the terminating zero
byte (assuming they cannot use xkb_keymap_new_from_file()).

This adds a new entry xkb_keymap_new_from_buffer() which takes a memory
location and the buffer size in bytes.

Internally, we depend on yy_scan_{string,byte}() helpers. According to
flex documentation these already copy the input string because they are
wrappers around yy_scan_buffer().
yy_scan_buffer() on the other hand has some insane requirements. The
buffer must be writeable and the last two bytes must be ASCII-NUL. But the
buffer may contain other 0 bytes just fine.

Because we don't want these constraints in our public API,
xkb_keymap_new_from_buffer() needs to create a copy of the input memory.
But it then calls yy_scan_buffer() directly. Hence, we have the same
number of buffer-copies as with *_from_string() but without the
terminating 0 requirement.
The explicit yy_scan_buffer() call is preferred over yy_scan_byte() so the
buffer-copy operation is not hidden somewhere in flex.

Maybe some day we no longer depend on flex and can have a zero-copy API. A
user could mmap() a file and it would get parsed right from this buffer.
But until then, we shouldn't expose this limitation in the API but instead
provide an API that some day can work with zero-copy.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

[ran: rebased on top of my branch]
Conflicts:
	Makefile.am
	src/xkbcomp/xkbcomp.c

1 files changed, 32 insertions, 0 deletions

diff --git a/src/xkbcomp/scanner.l b/src/xkbcomp/scanner.l
index 5ccf3e9..f30462d 100644
--- a/src/xkbcomp/scanner.l
+++ b/src/xkbcomp/scanner.l
@@ -267,6 +267,38 @@ XkbParseString(struct xkb_context *ctx, const char *string,
     return xkb_file;
 }
 
+/*
+ * yy_scan_buffer() requires the last two bytes of \buf to be 0. These two bytes
+ * are not scanned. Other zero bytes in the buffer are scanned normally, though.
+ * Due to these terminating zeroes, \length must be greater than 2.
+ * Furthermore, the buffer must be writable and you cannot make any assumptions
+ * about it after the scanner finished.
+ * All this must be guaranteed by the caller of this function!
+ */
+XkbFile *
+XkbParseBuffer(struct xkb_context *ctx, char *buf, size_t length,
+               const char *file_name)
+{
+    yyscan_t scanner;
+    struct scanner_extra extra;
+    YY_BUFFER_STATE state;
+    XkbFile *xkb_file;
+
+    if (!init_scanner(&scanner, &extra, ctx, file_name))
+        return NULL;
+
+    xkb_file = NULL;
+    state = yy_scan_buffer(buf, length, scanner);
+    if (state) {
+        xkb_file = parse(ctx, scanner, NULL);
+        yy_delete_buffer(state, scanner);
+    }
+
+    clear_scanner(scanner);
+
+    return xkb_file;
+}
+
 XkbFile *
 XkbParseFile(struct xkb_context *ctx, FILE *file,
              const char *file_name, const char *map)