Fix BoringSSL-GRPC template

templates/src/objective-c/BoringSSL-GRPC.podspec.template

@@ -2,13 +2,16 @@
 --- |
   <%!
     import subprocess, base64
-    import gzip
+    import gzip, shutil, os
+
     boringssl_commit = subprocess.check_output(['git', 'rev-parse', 'HEAD'], cwd='third_party/boringssl').decode().strip()
-    prefix_f = open("src/boringssl/boringssl_prefix_symbols.h", "rb")
-    prefix = prefix_f.read()
-    prefix_gzip = gzip.compress(prefix)
-    prefix_gzip_b64 = base64.b64encode(prefix_gzip)
-    prefix_f.close()
+    # TODO(mxyan): move to python3 style gzip compression when possible
+    with open('src/boringssl/boringssl_prefix_symbols.h', 'rb') as f_in, gzip.open('src/boringssl/boringssl_prefix_symbols.h.gz', 'wb') as f_out:
+      shutil.copyfileobj(f_in, f_out)
+    with open('src/boringssl/boringssl_prefix_symbols.h.gz', 'rb') as f_in:
+      prefix_gz = f_in.read()
+    os.remove('src/boringssl/boringssl_prefix_symbols.h.gz')
+    prefix_gz_b64 = base64.b64encode(prefix_gz)
   %>
 
   # This file has been automatically generated from a template file.
@@ -236,11 +239,11 @@
       # this moment. It has to be generated by BoringSSL's users and be injected to BoringSSL build.
       # gRPC generates this file in script /tools/distrib/upgrade_boringssl_objc.sh. This script
       # outputs a gzip+base64 encoded version of boringssl_prefix_symbols.h because of Cocoapods'
-      # limit on the 'prepare_command' field length. The encoded header is put at
-      # /src/boringssl/boringssl_prefix_symbols.h.gz.b64. Here we decode the content and inject
-      # the header to the correct location in BoringSSL.
+      # limit on the 'prepare_command' field length. The encoded header is generated from
+      # /src/boringssl/boringssl_prefix_symbols.h. Here we decode the content and inject the header to
+      # the correct location in BoringSSL.
       base64 -D <<EOF | gunzip > include/openssl/boringssl_prefix_symbols.h
-        ${prefix_gzip_b64}
+        ${prefix_gz_b64}
       EOF
 
       # We are renaming openssl to openssl_grpc so that there is no conflict with openssl if it exists