Add two-step write feature to bytebuf

If there is exactly one producer, then it can assume that the remaining
space in the buffer will only increase until it writes something.

This assumption may allow the producer to write to the buffer (up to a
known safe size) without any synchronization mechanism, thus allowing
to read and write different parts of the buffer in parallel.

The producer can then commit the write with a lock held, and update its
knowledge of the safe empty remaining space.

PR #3757 <https://github.com/Genymobile/scrcpy/pull/3757>

app/src/util/bytebuf.c

@@ -56,11 +56,9 @@ sc_bytebuf_skip(struct sc_bytebuf *buf, size_t len) {
     buf->tail = (buf->tail + len) % buf->alloc_size;
 }
 
-void
-sc_bytebuf_write(struct sc_bytebuf *buf, const uint8_t *from, size_t len) {
-    assert(len);
-    assert(len <= sc_bytebuf_write_available(buf));
-
+static inline void
+sc_bytebuf_write_step0(struct sc_bytebuf *buf, const uint8_t *from,
+                       size_t len) {
     size_t right_len = buf->alloc_size - buf->head;
     if (len < right_len) {
         right_len = len;
@@ -70,6 +68,37 @@ sc_bytebuf_write(struct sc_bytebuf *buf, const uint8_t *from, size_t len) {
     if (len > right_len) {
         memcpy(buf->data, from + right_len, len - right_len);
     }
+}
 
+static inline void
+sc_bytebuf_write_step1(struct sc_bytebuf *buf, size_t len) {
     buf->head = (buf->head + len) % buf->alloc_size;
 }
+
+void
+sc_bytebuf_write(struct sc_bytebuf *buf, const uint8_t *from, size_t len) {
+    assert(len);
+    assert(len <= sc_bytebuf_write_available(buf));
+
+    sc_bytebuf_write_step0(buf, from, len);
+    sc_bytebuf_write_step1(buf, len);
+}
+
+void
+sc_bytebuf_prepare_write(struct sc_bytebuf *buf, const uint8_t *from,
+                         size_t len) {
+    // *This function MUST NOT access buf->tail (even in assert()).*
+    // The purpose of this function is to allow a reader and a writer to access
+    // different parts of the buffer in parallel simultaneously. It is intended
+    // to be called without lock (only sc_bytebuf_commit_write() is intended to
+    // be called with lock held).
+
+    assert(len < buf->alloc_size - 1);
+    sc_bytebuf_write_step0(buf, from, len);
+}
+
+void
+sc_bytebuf_commit_write(struct sc_bytebuf *buf, size_t len) {
+    assert(len <= sc_bytebuf_write_available(buf));
+    sc_bytebuf_write_step1(buf, len);
+}

app/src/util/bytebuf.h

@@ -56,6 +56,30 @@ sc_bytebuf_skip(struct sc_bytebuf *buf, size_t len);
 void
 sc_bytebuf_write(struct sc_bytebuf *buf, const uint8_t *from, size_t len);
 
+/**
+ * Copy the user-provided array to the bytebuf, but do not advance the cursor
+ *
+ * The caller must check that len <= sc_bytebuf_write_available() (it is an
+ * error to write more bytes than the remaining available space).
+ *
+ * After this function is called, the write must be committed with
+ * sc_bytebuf_commit_write().
+ *
+ * The purpose of this mechanism is to acquire a lock only to commit the write,
+ * but not to perform the actual copy.
+ *
+ * This function is guaranteed not to access buf->tail.
+ */
+void
+sc_bytebuf_prepare_write(struct sc_bytebuf *buf, const uint8_t *from,
+                         size_t len);
+
+/**
+ * Commit a prepared write
+ */
+void
+sc_bytebuf_commit_write(struct sc_bytebuf *buf, size_t len);
+
 /**
  * Return the number of bytes which can be read
  *

app/tests/test_bytebuf.c

@@ -71,12 +71,56 @@ void test_bytebuf_boundaries(void) {
     sc_bytebuf_destroy(&buf);
 }
 
+void test_bytebuf_two_steps_write(void) {
+    struct sc_bytebuf buf;
+    uint8_t data[20];
+
+    bool ok = sc_bytebuf_init(&buf, 20);
+    assert(ok);
+
+    sc_bytebuf_write(&buf, (uint8_t *) "hello ", sizeof("hello ") - 1);
+    assert(sc_bytebuf_read_available(&buf) == 6);
+
+    sc_bytebuf_write(&buf, (uint8_t *) "hello ", sizeof("hello ") - 1);
+    assert(sc_bytebuf_read_available(&buf) == 12);
+
+    sc_bytebuf_prepare_write(&buf, (uint8_t *) "hello ", sizeof("hello ") - 1);
+    assert(sc_bytebuf_read_available(&buf) == 12); // write not committed yet
+
+    sc_bytebuf_read(&buf, data, 9);
+    assert(!strncmp((char *) data, "hello hel", 3));
+    assert(sc_bytebuf_read_available(&buf) == 3);
+
+    sc_bytebuf_commit_write(&buf, sizeof("hello ") - 1);
+    assert(sc_bytebuf_read_available(&buf) == 9);
+
+    sc_bytebuf_prepare_write(&buf, (uint8_t *) "world", sizeof("world") - 1);
+    assert(sc_bytebuf_read_available(&buf) == 9); // write not committed yet
+
+    sc_bytebuf_commit_write(&buf, sizeof("world") - 1);
+    assert(sc_bytebuf_read_available(&buf) == 14);
+
+    sc_bytebuf_write(&buf, (uint8_t *) "!", 1);
+    assert(sc_bytebuf_read_available(&buf) == 15);
+
+    sc_bytebuf_skip(&buf, 3);
+    assert(sc_bytebuf_read_available(&buf) == 12);
+
+    sc_bytebuf_read(&buf, data, 12);
+    data[12] = '\0';
+    assert(!strcmp((char *) data, "hello world!"));
+    assert(sc_bytebuf_read_available(&buf) == 0);
+
+    sc_bytebuf_destroy(&buf);
+}
+
 int main(int argc, char *argv[]) {
     (void) argc;
     (void) argv;
 
     test_bytebuf_simple();
     test_bytebuf_boundaries();
+    test_bytebuf_two_steps_write();
 
     return 0;
 }