reftable/basics: provide new reftable_buf interface

Implement a new `reftable_buf` interface that will replace Git's own
`strbuf` interface. This is done due to three reasons:

  - The `strbuf` interfaces do not handle memory allocation failures and
    instead causes us to die. This is okay in the context of Git, but is
    not in the context of the reftable library, which is supposed to be
    usable by third-party applications.

  - The `strbuf` interface is quite deeply tied into Git, which makes it
    hard to use the reftable library as a standalone library. Any
    dependent would have to carefully extract the relevant parts of it
    to make things work, which is not all that sensible.

  - The `strbuf` interface does not use the pluggable allocators that
    can be set up via `reftable_set_alloc()`.

So we have good reasons to use our own type, and the implementation is
rather trivial. Implement our own type. Conversion of the reftable
library will be handled in subsequent commits.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Taylor Blau <[email protected]>

Author: pks-t

reftable/basics.c

@@ -9,6 +9,7 @@ license that can be found in the LICENSE file or at
 #define REFTABLE_ALLOW_BANNED_ALLOCATORS
 #include "basics.h"
 #include "reftable-basics.h"
+#include "reftable-error.h"
 
 static void *(*reftable_malloc_ptr)(size_t sz);
 static void *(*reftable_realloc_ptr)(void *, size_t);
@@ -69,6 +70,79 @@ void reftable_set_alloc(void *(*malloc)(size_t),
 	reftable_free_ptr = free;
 }
 
+void reftable_buf_init(struct reftable_buf *buf)
+{
+	struct reftable_buf empty = REFTABLE_BUF_INIT;
+	*buf = empty;
+}
+
+void reftable_buf_release(struct reftable_buf *buf)
+{
+	reftable_free(buf->buf);
+	reftable_buf_init(buf);
+}
+
+void reftable_buf_reset(struct reftable_buf *buf)
+{
+	if (buf->alloc) {
+		buf->len = 0;
+		buf->buf[0] = '\0';
+	}
+}
+
+int reftable_buf_setlen(struct reftable_buf *buf, size_t len)
+{
+	if (len > buf->len)
+		return -1;
+	if (len == buf->len)
+		return 0;
+	buf->buf[len] = '\0';
+	buf->len = len;
+	return 0;
+}
+
+int reftable_buf_cmp(const struct reftable_buf *a, const struct reftable_buf *b)
+{
+	size_t len = a->len < b->len ? a->len : b->len;
+	if (len) {
+		int cmp = memcmp(a->buf, b->buf, len);
+		if (cmp)
+			return cmp;
+	}
+	return a->len < b->len ? -1 : a->len != b->len;
+}
+
+int reftable_buf_add(struct reftable_buf *buf, const void *data, size_t len)
+{
+	size_t newlen = buf->len + len;
+
+	if (newlen + 1 > buf->alloc) {
+		char *reallocated = buf->buf;
+		REFTABLE_ALLOC_GROW(reallocated, newlen + 1, buf->alloc);
+		if (!reallocated)
+			return REFTABLE_OUT_OF_MEMORY_ERROR;
+		buf->buf = reallocated;
+	}
+
+	memcpy(buf->buf + buf->len, data, len);
+	buf->buf[newlen] = '\0';
+	buf->len = newlen;
+
+	return 0;
+}
+
+int reftable_buf_addstr(struct reftable_buf *buf, const char *s)
+{
+	return reftable_buf_add(buf, s, strlen(s));
+}
+
+char *reftable_buf_detach(struct reftable_buf *buf)
+{
+	char *result = buf->buf;
+	reftable_buf_init(buf);
+	return result;
+}
+
 void put_be24(uint8_t *out, uint32_t i)
 {
 	out[0] = (uint8_t)((i >> 16) & 0xff);

reftable/basics.h

@@ -16,6 +16,62 @@ license that can be found in the LICENSE file or at
 #include "system.h"
 #include "reftable-basics.h"
 
+struct reftable_buf {
+	size_t alloc;
+	size_t len;
+	char *buf;
+};
+#define REFTABLE_BUF_INIT { 0 }
+
+/*
+ * Initialize the buffer such that it is ready for use. This is equivalent to
+ * using REFTABLE_BUF_INIT for stack-allocated variables.
+ */
+void reftable_buf_init(struct reftable_buf *buf);
+
+/*
+ * Release memory associated with the buffer. The buffer is reinitialized such
+ * that it can be reused for subsequent operations.
+ */
+void reftable_buf_release(struct reftable_buf *buf);
+
+/*
+ * Reset the buffer such that it is effectively empty, without releasing the
+ * memory that this structure holds on to. This is equivalent to calling
+ * `reftable_buf_setlen(buf, 0)`.
+ */
+void reftable_buf_reset(struct reftable_buf *buf);
+
+/*
+ * Trim the buffer to a shorter length by updating the `len` member and writing
+ * a NUL byte to `buf[len]`. Returns 0 on success, -1 when `len` points outside
+ * of the array.
+ */
+int reftable_buf_setlen(struct reftable_buf *buf, size_t len);
+
+/*
+ * Lexicographically compare the two buffers. Returns 0 when both buffers have
+ * the same contents, -1 when `a` is lexicographically smaller than `b`, and 1
+ * otherwise.
+ */
+int reftable_buf_cmp(const struct reftable_buf *a, const struct reftable_buf *b);
+
+/*
+ * Add the given bytes to the buffer. Returns 0 on success,
+ * REFTABLE_OUT_OF_MEMORY_ERROR on allocation failure.
+ */
+int reftable_buf_add(struct reftable_buf *buf, const void *data, size_t len);
+
+/* Equivalent to `reftable_buf_add(buf, s, strlen(s))`. */
+int reftable_buf_addstr(struct reftable_buf *buf, const char *s);
+
+/*
+ * Detach the buffer from the structure such that the underlying memory is now
+ * owned by the caller. The buffer is reinitialized such that it can be reused
+ * for subsequent operations.
+ */
+char *reftable_buf_detach(struct reftable_buf *buf);
+
 /* Bigendian en/decoding of integers */
 
 void put_be24(uint8_t *out, uint32_t i);