update extract_kernel.py

commit 8b727765111bde78d1b1174e0ed54db819605dd8
Author: Yifan Hong <elsk@google.com>
Date:   Wed Sep 4 12:40:13 2019 -0700

    extract-kernel: Fix indexing

    The original script always tries to extract from index
    0 even if header is found in positive indices. Fix that.

    Also, continue to try other positions if previous run failed.

    Test: builds
    Bug: 139348603

    Change-Id: Ia54fc709de9ae587fc64b59d94a1fa4ae669c14f

external/extract_kernel.py

@@ -47,7 +47,10 @@ def get_version(input_bytes, start_idx):
   null_idx = input_bytes.find('\x00', start_idx)
   if null_idx < 0:
     return None
+  try:
     linux_banner = input_bytes[start_idx:null_idx].decode()
+  except UnicodeDecodeError:
+    return None
   mo = re.match(LINUX_BANNER_REGEX, linux_banner)
   if mo:
     return mo.group(1)
@@ -97,19 +100,25 @@ def dump_configs(input_bytes):
   return o
 
 
-def try_decompress(cmd, search_bytes, input_bytes):
-  idx = input_bytes.find(search_bytes)
-  if idx < 0:
-    return None
-
-  idx = 0
+def try_decompress_bytes(cmd, input_bytes):
   sp = subprocess.Popen(cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE,
                         stderr=subprocess.PIPE)
-  o, _ = sp.communicate(input=input_bytes[idx:])
+  o, _ = sp.communicate(input=input_bytes)
   # ignore errors
   return o
 
 
+def try_decompress(cmd, search_bytes, input_bytes):
+  idx = 0
+  while True:
+    idx = input_bytes.find(search_bytes, idx)
+    if idx < 0:
+      raise StopIteration()
+
+    yield try_decompress_bytes(cmd, input_bytes[idx:])
+    idx += 1
+
+
 def decompress_dump(func, input_bytes):
   """
   Run func(input_bytes) first; and if that fails (returns value evaluates to
@@ -119,15 +128,15 @@ def decompress_dump(func, input_bytes):
   if o:
     return o
   for cmd, search_bytes in COMPRESSION_ALGO:
-    decompressed = try_decompress(cmd, search_bytes, input_bytes)
+    for decompressed in try_decompress(cmd, search_bytes, input_bytes):
       if decompressed:
-      o = func(decompressed)
+        o = decompress_dump(func, decompressed)
         if o:
           return o
     # Force decompress the whole file even if header doesn't match
-    decompressed = try_decompress(cmd, b"", input_bytes)
+    decompressed = try_decompress_bytes(cmd, input_bytes)
     if decompressed:
-      o = func(decompressed)
+      o = decompress_dump(func, decompressed)
       if o:
         return o