streaming: a new API to read from the object store

Given an object name, use open_istream() to get a git_istream handle
that you can read_istream() from as if you are using read(2) to read
the contents of the object, and close it with close_istream() when
you are done.

Currently, we do not do anything fancy--it just calls read_sha1_file()
and keeps the contents in memory as a whole, and carve it out as you
request with read_istream().

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -552,6 +552,7 @@ LIB_H += sha1-lookup.h
 LIB_H += sideband.h
 LIB_H += sigchain.h
 LIB_H += strbuf.h
+LIB_H += streaming.h
 LIB_H += string-list.h
 LIB_H += submodule.h
 LIB_H += tag.h
@@ -657,6 +658,7 @@ LIB_OBJS += shallow.o
 LIB_OBJS += sideband.o
 LIB_OBJS += sigchain.o
 LIB_OBJS += strbuf.o
+LIB_OBJS += streaming.o
 LIB_OBJS += string-list.o
 LIB_OBJS += submodule.o
 LIB_OBJS += symlinks.o

streaming.c

@@ -0,0 +1,193 @@
+/*
+ * Copyright (c) 2011, Google Inc.
+ */
+#include "cache.h"
+#include "streaming.h"
+
+enum input_source {
+	stream_error = -1,
+	incore = 0,
+	loose = 1,
+	pack_non_delta = 2
+};
+
+typedef int (*open_istream_fn)(struct git_istream *,
+			       struct object_info *,
+			       const unsigned char *,
+			       enum object_type *);
+typedef int (*close_istream_fn)(struct git_istream *);
+typedef ssize_t (*read_istream_fn)(struct git_istream *, char *, size_t);
+
+struct stream_vtbl {
+	close_istream_fn close;
+	read_istream_fn read;
+};
+
+#define open_method_decl(name) \
+	int open_istream_ ##name \
+	(struct git_istream *st, struct object_info *oi, \
+	 const unsigned char *sha1, \
+	 enum object_type *type)
+
+#define close_method_decl(name) \
+	int close_istream_ ##name \
+	(struct git_istream *st)
+
+#define read_method_decl(name) \
+	ssize_t read_istream_ ##name \
+	(struct git_istream *st, char *buf, size_t sz)
+
+/* forward declaration */
+static open_method_decl(incore);
+static open_method_decl(loose);
+static open_method_decl(pack_non_delta);
+
+static open_istream_fn open_istream_tbl[] = {
+	open_istream_incore,
+	open_istream_loose,
+	open_istream_pack_non_delta,
+};
+
+struct git_istream {
+	const struct stream_vtbl *vtbl;
+	unsigned long size; /* inflated size of full object */
+
+	union {
+		struct {
+			char *buf; /* from read_object() */
+			unsigned long read_ptr;
+		} incore;
+
+		struct {
+			int fd; /* open for reading */
+			/* NEEDSWORK: what else? */
+		} loose;
+
+		struct {
+			int fd; /* open for reading */
+			/* NEEDSWORK: what else? */
+		} in_pack;
+	} u;
+};
+
+int close_istream(struct git_istream *st)
+{
+	return st->vtbl->close(st);
+}
+
+ssize_t read_istream(struct git_istream *st, char *buf, size_t sz)
+{
+	return st->vtbl->read(st, buf, sz);
+}
+
+static enum input_source istream_source(const unsigned char *sha1,
+					enum object_type *type,
+					struct object_info *oi)
+{
+	unsigned long size;
+	int status;
+
+	oi->sizep = &size;
+	status = sha1_object_info_extended(sha1, oi);
+	if (status < 0)
+		return stream_error;
+	*type = status;
+
+	switch (oi->whence) {
+	case OI_LOOSE:
+		return loose;
+	case OI_PACKED:
+		if (!oi->u.packed.is_delta && big_file_threshold <= size)
+			return pack_non_delta;
+		/* fallthru */
+	default:
+		return incore;
+	}
+}
+
+struct git_istream *open_istream(const unsigned char *sha1,
+				 enum object_type *type,
+				 unsigned long *size)
+{
+	struct git_istream *st;
+	struct object_info oi;
+	const unsigned char *real = lookup_replace_object(sha1);
+	enum input_source src = istream_source(real, type, &oi);
+
+	if (src < 0)
+		return NULL;
+
+	st = xmalloc(sizeof(*st));
+	if (open_istream_tbl[src](st, &oi, real, type)) {
+		if (open_istream_incore(st, &oi, real, type)) {
+			free(st);
+			return NULL;
+		}
+	}
+	*size = st->size;
+	return st;
+}
+
+/*****************************************************************
+ *
+ * Loose object stream
+ *
+ *****************************************************************/
+
+static open_method_decl(loose)
+{
+	return -1; /* for now */
+}
+
+
+/*****************************************************************
+ *
+ * Non-delta packed object stream
+ *
+ *****************************************************************/
+
+static open_method_decl(pack_non_delta)
+{
+	return -1; /* for now */
+}
+
+
+/*****************************************************************
+ *
+ * In-core stream
+ *
+ *****************************************************************/
+
+static close_method_decl(incore)
+{
+	free(st->u.incore.buf);
+	return 0;
+}
+
+static read_method_decl(incore)
+{
+	size_t read_size = sz;
+	size_t remainder = st->size - st->u.incore.read_ptr;
+
+	if (remainder <= read_size)
+		read_size = remainder;
+	if (read_size) {
+		memcpy(buf, st->u.incore.buf + st->u.incore.read_ptr, read_size);
+		st->u.incore.read_ptr += read_size;
+	}
+	return read_size;
+}
+
+static struct stream_vtbl incore_vtbl = {
+	close_istream_incore,
+	read_istream_incore,
+};
+
+static open_method_decl(incore)
+{
+	st->u.incore.buf = read_sha1_file_extended(sha1, type, &st->size, 0);
+	st->u.incore.read_ptr = 0;
+	st->vtbl = &incore_vtbl;
+
+	return st->u.incore.buf ? 0 : -1;
+}

streaming.h

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2011, Google Inc.
+ */
+#ifndef STREAMING_H
+#define STREAMING_H 1
+#include "cache.h"
+
+/* opaque */
+struct git_istream;
+
+extern struct git_istream *open_istream(const unsigned char *, enum object_type *, unsigned long *);
+extern int close_istream(struct git_istream *);
+extern ssize_t read_istream(struct git_istream *, char *, size_t);
+
+#endif /* STREAMING_H */