Relax constraints on Y4M header parsing

Previous parser assumed that the header would not exceed
80 characters. However, with latest FFMPEG changes, the header
of Y4M files can exceed this limit.

New parser can parse an arbitrarily long header, as long each
tag is 255 or less characters.

BUG=aomedia:2876

Change-Id: Icdd0f03bb9f2a6402a5a9c42cd5d1256a9c56f66

common/y4minput.c

diff --git a/common/y4minput.c b/common/y4minput.c
index d537197..9d83505 100644
--- a/common/y4minput.c
+++ b/common/y4minput.c
@@ -96,11 +96,50 @@
   return 0;
 }
 
+// Copy a single tag into the buffer, along with a null character.
+// Returns 0 if any file IO errors occur.
+static int copy_tag(char *buf, size_t buf_len, char *end_tag, FILE *file) {
+  size_t i;
+  assert(buf_len >= 1);
+  // Skip leading space characters.
+  do {
+    if (!file_read(buf, 1, file)) {
+      return 0;
+    }
+  } while (buf[0] == ' ');
+
+  // If we hit the newline, treat this as the "empty" tag.
+  if (buf[0] == '\n') {
+    buf[0] = '\0';
+    *end_tag = '\n';
+    return 1;
+  }
+
+  // Copy over characters until a space is hit, or the buffer is exhausted.
+  for (i = 1; i < buf_len; ++i) {
+    if (!file_read(buf + i, 1, file)) {
+      return 0;
+    }
+    if (buf[i] == ' ' || buf[i] == '\n') {
+      break;
+    }
+  }
+  if (i == buf_len) {
+    fprintf(stderr, "Error: Y4M header tags must be less than %lu characters\n",
+            (unsigned long)i);
+    return 0;
+  }
+  *end_tag = buf[i];
+  buf[i] = '\0';
+  return 1;
+}
+
 // Returns 1 if tags were parsed successfully, 0 otherwise.
-static int parse_tags(y4m_input *y4m_ctx, char *buffer) {
+static int parse_tags(y4m_input *y4m_ctx, FILE *file) {
+  char tag[256];
+  char end;  // Character denoting the end of the tag, ' ' or '\n'.
   // Set Y4M tags to defaults, updating them as processing occurs. Mandatory
   // fields are marked with -1 and will be checked after the tags are parsed.
-  int ret;
   y4m_ctx->pic_w = -1;
   y4m_ctx->pic_h = -1;
   y4m_ctx->fps_n = -1;  // Also serves as marker for fps_d
@@ -109,10 +148,16 @@
   y4m_ctx->interlace = '?';
   snprintf(y4m_ctx->chroma_type, sizeof(y4m_ctx->chroma_type), "420");
 
-  ret = y4m_parse_tags(y4m_ctx, buffer);
-  if (ret < 0) {
-    return 0;
-  }
+  // Find one tag at a time.
+  do {
+    if (!copy_tag(tag, sizeof(tag), &end, file)) {
+      return 0;
+    }
+    // y4m_parse_tags returns 0 on success.
+    if (y4m_parse_tags(y4m_ctx, tag)) {
+      return 0;
+    }
+  } while (end != '\n');
 
   // Check the mandatory fields.
   if (y4m_ctx->pic_w == -1) {
@@ -802,8 +847,7 @@
                    int num_skip, aom_chroma_sample_position_t csp,
                    int only_420) {
   // File must start with |TAG|.
-  char tag_buffer[9];                        // 9 == strlen(TAG)
-  char buffer[Y4M_HEADER_BUF_SIZE] = { 0 };  // Rest of header.
+  char tag_buffer[9];  // 9 == strlen(TAG)
   // Read as much as possible from |skip_buffer|, which were characters
   // that were previously read from the file to do input-type detection.
   assert(num_skip >= 0 && num_skip <= 8);
@@ -819,23 +863,12 @@
     return -1;
   }
   // Next character must be a space.
-  if (!file_read(buffer, 1, file) || buffer[0] != ' ') {
+  if (!file_read(tag_buffer, 1, file) || tag_buffer[0] != ' ') {
     fprintf(stderr, "Error parsing header: space must follow %s\n", TAG);
     return -1;
   }
-  int i;
-  /*Read until newline, or Y4M_HEADER_BUF_SIZE cols, whichever happens first.*/
-  for (i = 0; i < Y4M_HEADER_BUF_SIZE - 1; i++) {
-    if (!file_read(buffer + i, 1, file)) return -1;
-    if (buffer[i] == '\n') break;
-  }
-  if (i == Y4M_HEADER_BUF_SIZE - 1) {
-    fprintf(stderr, "Error parsing header; not a YUV4MPEG2 file?\n");
-    return -1;
-  }
-  buffer[i] = '\0';
-  if (!parse_tags(y4m_ctx, buffer)) {
-    fprintf(stderr, "Error parsing YUV4MPEG2 header.\n");
+  if (!parse_tags(y4m_ctx, file)) {
+    fprintf(stderr, "Error parsing %s header.\n", TAG);
     return -1;
   }
   if (y4m_ctx->interlace == '?') {