stream: Implement slice:// for reading slices of streams

 Add support for reading a byte range from a stream via
 the `slice://` protocol.

 Syntax is `slice://start[-end]@URL` where end is a maximum
 (read until end or eof).

 Size suffixes support in `m_option` is reused so they can
 be used with start/end.

 This can be very useful with e.g. large MPEGTS streams with
 corruption or time-stamp jumps or other issues in them.

Signed-off-by: Mohammad AlSaleh <CE.Mohammad.AlSaleh@gmail.com>

4 files changed, 200 insertions, 0 deletions

diff --git a/DOCS/man/mpv.rst b/DOCS/man/mpv.rst
index b5739d7707..dc61394ff7 100644
--- a/DOCS/man/mpv.rst
+++ b/DOCS/man/mpv.rst
@@ -1194,6 +1194,27 @@ PROTOCOLS
 
     Stitch together parts of multiple files and play them.
 
+``slice://start[-end]@URL``
+
+    Read a slice of a stream.
+
+    ``start`` and ``end`` represent a byte range and accept
+    suffixes such as ``KiB`` and ``MiB``. ``end`` is optional.
+
+    Only works with seekable streams.
+
+    Examples::
+
+      mpv slice://1g-2g@cap.ts
+
+      This starts reading from cap.ts after seeking 1 GiB, then
+      reads until reaching 2 GiB or end of file.
+
+      mpv slice://100m@appending://cap.ts
+
+      This starts reading from cap.ts after seeking 100MiB, then
+      reads until end of file.
+
 ``null://``
 
     Simulate an empty file. If opened for writing, it will discard all data.
diff --git a/stream/stream.c b/stream/stream.c
index 3c728eacac..f422c5d08d 100644
--- a/stream/stream.c
+++ b/stream/stream.c
@@ -51,6 +51,7 @@ extern const stream_info_t stream_info_ffmpeg;
 extern const stream_info_t stream_info_ffmpeg_unsafe;
 extern const stream_info_t stream_info_avdevice;
 extern const stream_info_t stream_info_file;
+extern const stream_info_t stream_info_slice;
 extern const stream_info_t stream_info_fd;
 extern const stream_info_t stream_info_ifo_dvdnav;
 extern const stream_info_t stream_info_dvdnav;
@@ -88,6 +89,7 @@ static const stream_info_t *const stream_list[] = {
     &stream_info_mf,
     &stream_info_edl,
     &stream_info_file,
+    &stream_info_slice,
     &stream_info_fd,
     &stream_info_cb,
     NULL
diff --git a/stream/stream_slice.c b/stream/stream_slice.c
new file mode 100644
index 0000000000..7889c8d059
--- /dev/null
+++ b/stream/stream_slice.c
@@ -0,0 +1,176 @@
+/*
+ * This file is part of mpv.
+ *
+ * mpv is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * mpv is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with mpv.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <libavutil/common.h>
+#include <stdint.h>
+#include <string.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "common/common.h"
+#include "options/m_option.h"
+#include "options/path.h"
+#include "stream.h"
+
+struct priv {
+    int64_t slice_start;
+    int64_t slice_max_end; // 0 for no limit
+    struct stream *inner;
+};
+
+static int fill_buffer(struct stream *s, void *buffer, int len)
+{
+    struct priv *p = s->priv;
+
+    if (p->slice_max_end) {
+        // We don't simply use (s->pos >= size) to avoid early return
+        // if the file is still being appended to.
+        if (s->pos + p->slice_start >= p->slice_max_end)
+            return -1;
+        // Avoid rading beyond p->slice_max_end
+        len = MPMIN(len, p->slice_max_end - s->pos);
+    }
+
+    return stream_read_partial(p->inner, buffer, len);
+}
+
+static int seek(struct stream *s, int64_t newpos)
+{
+    struct priv *p = s->priv;
+    return stream_seek(p->inner, newpos + p->slice_start);
+}
+
+static int64_t get_size(struct stream *s)
+{
+    struct priv *p = s->priv;
+    int64_t size = stream_get_size(p->inner);
+    if (size <= 0)
+        return size;
+    if (size <= p->slice_start)
+      return 0;
+    if (p->slice_max_end)
+        size = MPMIN(size, p->slice_max_end);
+    return size - p->slice_start;
+}
+
+static void s_close(struct stream *s)
+{
+    struct priv *p = s->priv;
+    free_stream(p->inner);
+}
+
+static int parse_slice_range(stream_t *stream)
+{
+    struct priv *p = stream->priv;
+
+    struct bstr b_url = bstr0(stream->url);
+    struct bstr proto_with_range, inner_url;
+
+    bool has_at = bstr_split_tok(b_url, "@", &proto_with_range, &inner_url);
+
+    if (!has_at) {
+        MP_ERR(stream, "Expected slice://start[-end]@URL: '%s'\n", stream->url);
+        return STREAM_ERROR;
+    }
+
+    if (!inner_url.len) {
+        MP_ERR(stream, "URL expected to follow 'slice://start[-end]@': '%s'.\n", stream->url);
+        return STREAM_ERROR;
+    }
+    stream->path = bstrto0(stream, inner_url);
+
+    mp_split_proto(proto_with_range, &proto_with_range);
+    struct bstr range = proto_with_range;
+
+    struct bstr start, end;
+    bool has_end = bstr_split_tok(range, "-", &start, &end);
+
+    if (!start.len) {
+        MP_ERR(stream, "The byte range must have a start, and it can't be negative: '%s'\n", stream->url);
+        return STREAM_ERROR;
+    }
+
+    if (has_end && !end.len) {
+        MP_ERR(stream, "The byte range end can be omitted, but it can't be empty: '%s'\n", stream->url);
+        return STREAM_ERROR;
+    }
+
+    const struct m_option opt = {
+        .type = &m_option_type_byte_size,
+    };
+
+    if (m_option_parse(stream->log, &opt, bstr0("slice_start"), start, &p->slice_start) < 0)
+        return STREAM_ERROR;
+
+    if (has_end) {
+        if (m_option_parse(stream->log, &opt, bstr0("slice_max_end"), end, &p->slice_max_end) < 0)
+            return STREAM_ERROR;
+    }
+
+    if (p->slice_max_end && p->slice_max_end < p->slice_start) {
+        MP_ERR(stream, "The byte range end (%"PRId64") can't be smaller than the start (%"PRId64"): '%s'\n",
+                p->slice_max_end,
+                p->slice_start,
+                stream->url);
+        return STREAM_ERROR;
+    }
+
+    return STREAM_OK;
+}
+
+static int open2(struct stream *stream, struct stream_open_args *args)
+{
+    struct priv *p = talloc_zero(stream, struct priv);
+    stream->priv = p;
+
+    stream->fill_buffer = fill_buffer;
+    stream->seek = seek;
+    stream->get_size = get_size;
+    stream->close = s_close;
+
+    int parse_ret = parse_slice_range(stream);
+    if (parse_ret != STREAM_OK) {
+        return parse_ret;
+    }
+
+    args->url = stream->path;
+    int inner_ret = stream_create_with_args(args, &p->inner);
+    if (inner_ret != STREAM_OK) {
+        return inner_ret;
+    }
+
+    if (!p->inner->seekable) {
+        MP_FATAL(stream, "Non-seekable stream '%s' can't be used with 'slice://'\n", p->inner->url);
+        free_stream(p->inner);
+        return STREAM_ERROR;
+    }
+
+    stream->seekable = 1;
+    stream->stream_origin = p->inner->stream_origin;
+
+    if (p->slice_start)
+        seek(stream, 0);
+
+    return STREAM_OK;
+}
+
+const stream_info_t stream_info_slice = {
+    .name = "slice",
+    .open2 = open2,
+    .protocols = (const char*const[]){ "slice", NULL },
+    .can_write = false,
+};
diff --git a/wscript_build.py b/wscript_build.py
index c57619b353..41c8e60f27 100644
--- a/wscript_build.py
+++ b/wscript_build.py
@@ -360,6 +360,7 @@ def build(ctx):
         ( "stream/stream_cb.c" ),
         ( "stream/stream_cdda.c",                "cdda" ),
         ( "stream/stream_concat.c" ),
+        ( "stream/stream_slice.c" ),
         ( "stream/stream_dvb.c",                 "dvbin" ),
         ( "stream/stream_dvdnav.c",              "dvdnav" ),
         ( "stream/stream_edl.c" ),