Pre-allocate, check, and consume method, scheme, and content-type headers at
the server.

Verify that method, scheme, and te trailers are present, and error out if not.
Complain if content-type doesn't match the format, but don't error out.

This currently, for now, blindly allows all three schemes (grpc, http, https)
without verification, although that will change once the C implementation
finishes switching to http/https.

Cloned from CL 82558661 by 'g4 patch'.
Original change by klempner@klempner:grpc_metadata:808:citc on 2014/12/19 18:41:47.
	Change on 2015/01/12 by ctiller <ctiller@google.com>
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=83785251
diff --git a/src/core/channel/http_server_filter.c b/src/core/channel/http_server_filter.c
index 9a20d79..44eab43 100644
--- a/src/core/channel/http_server_filter.c
+++ b/src/core/channel/http_server_filter.c
@@ -32,13 +32,26 @@
  */
 
 #include "src/core/channel/http_server_filter.h"
+
+#include <string.h>
 #include <grpc/support/log.h>
 
-typedef struct call_data { int sent_status; } call_data;
+typedef struct call_data {
+  int sent_status;
+  int seen_scheme;
+  int seen_method;
+  int seen_te_trailers;
+} call_data;
 
 typedef struct channel_data {
   grpc_mdelem *te_trailers;
-  grpc_mdelem *status_md;
+  grpc_mdelem *method;
+  grpc_mdelem *http_scheme;
+  grpc_mdelem *https_scheme;
+  /* TODO(klempner): Remove this once we stop using it */
+  grpc_mdelem *grpc_scheme;
+  grpc_mdelem *content_type;
+  grpc_mdelem *status;
 } channel_data;
 
 /* used to silence 'variable not used' warnings */
@@ -56,20 +69,54 @@
   channel_data *channeld = elem->channel_data;
   GRPC_CALL_LOG_OP(GPR_INFO, elem, op);
 
-  ignore_unused(calld);
-  ignore_unused(channeld);
-
   switch (op->type) {
     case GRPC_RECV_METADATA:
-      /* check if it's a te: trailers header */
-      if (op->data.metadata == channeld->te_trailers) {
+      /* Check if it is one of the headers we care about. */
+      if (op->data.metadata == channeld->te_trailers ||
+          op->data.metadata == channeld->method ||
+          op->data.metadata == channeld->http_scheme ||
+          op->data.metadata == channeld->https_scheme ||
+          op->data.metadata == channeld->grpc_scheme ||
+          op->data.metadata == channeld->content_type) {
         /* swallow it */
+        if (op->data.metadata == channeld->method) {
+          calld->seen_method = 1;
+        } else if (op->data.metadata->key == channeld->http_scheme->key) {
+          calld->seen_scheme = 1;
+        } else if (op->data.metadata == channeld->te_trailers) {
+          calld->seen_te_trailers = 1;
+        }
+        /* TODO(klempner): Track that we've seen all the headers we should
+           require */
         grpc_mdelem_unref(op->data.metadata);
         op->done_cb(op->user_data, GRPC_OP_OK);
-      } else if (op->data.metadata->key == channeld->te_trailers->key) {
-        gpr_log(GPR_ERROR, "Invalid te: header: '%s'",
+      } else if (op->data.metadata->key == channeld->content_type->key) {
+        if (strncmp(grpc_mdstr_as_c_string(op->data.metadata->value),
+                    "application/grpc+", 17) == 0) {
+          /* Although the C implementation doesn't (currently) generate them,
+             any
+             custom +-suffix is explicitly valid. */
+          /* TODO(klempner): We should consider preallocating common values such
+             as +proto or +json, or at least stashing them if we see them. */
+          /* TODO(klempner): Should we be surfacing this to application code? */
+        } else {
+          /* TODO(klempner): We're currently allowing this, but we shouldn't
+             see it without a proxy so log for now. */
+          gpr_log(GPR_INFO, "Unexpected content-type %s",
+                  channeld->content_type->key);
+        }
+        grpc_mdelem_unref(op->data.metadata);
+        op->done_cb(op->user_data, GRPC_OP_OK);
+      } else if (op->data.metadata->key == channeld->te_trailers->key ||
+                 op->data.metadata->key == channeld->method->key ||
+                 op->data.metadata->key == channeld->http_scheme->key ||
+                 op->data.metadata->key == channeld->content_type->key) {
+        gpr_log(GPR_ERROR, "Invalid %s: header: '%s'",
+                grpc_mdstr_as_c_string(op->data.metadata->key),
                 grpc_mdstr_as_c_string(op->data.metadata->value));
-        /* swallow it */
+        /* swallow it and error everything out. */
+        /* TODO(klempner): We ought to generate more descriptive error messages
+           on the wire here. */
         grpc_mdelem_unref(op->data.metadata);
         op->done_cb(op->user_data, GRPC_OP_OK);
         grpc_call_element_send_cancel(elem);
@@ -78,14 +125,33 @@
         grpc_call_next_op(elem, op);
       }
       break;
+    case GRPC_RECV_END_OF_INITIAL_METADATA:
+      /* Have we seen the required http2 transport headers?
+         (:method, :scheme, content-type, with :path and :authority covered
+         at the channel level right now) */
+      if (calld->seen_method && calld->seen_scheme && calld->seen_te_trailers) {
+        grpc_call_next_op(elem, op);
+      } else {
+        if (!calld->seen_method) {
+          gpr_log(GPR_ERROR, "Missing :method header");
+        } else if (!calld->seen_scheme) {
+          gpr_log(GPR_ERROR, "Missing :scheme header");
+        } else if (!calld->seen_te_trailers) {
+          gpr_log(GPR_ERROR, "Missing te trailers header");
+        }
+        /* Error this call out */
+        op->done_cb(op->user_data, GRPC_OP_OK);
+        grpc_call_element_send_cancel(elem);
+      }
+      break;
     case GRPC_SEND_START:
     case GRPC_SEND_METADATA:
       /* If we haven't sent status 200 yet, we need to so so because it needs to
          come before any non : prefixed metadata. */
       if (!calld->sent_status) {
         calld->sent_status = 1;
-        /* status_md is reffed by grpc_call_element_send_metadata */
-        grpc_call_element_send_metadata(elem, channeld->status_md);
+        /* status is reffed by grpc_call_element_send_metadata */
+        grpc_call_element_send_metadata(elem, channeld->status);
       }
       grpc_call_next_op(elem, op);
       break;
@@ -124,6 +190,9 @@
 
   /* initialize members */
   calld->sent_status = 0;
+  calld->seen_scheme = 0;
+  calld->seen_method = 0;
+  calld->seen_te_trailers = 0;
 }
 
 /* Destructor for call_data */
@@ -151,7 +220,13 @@
 
   /* initialize members */
   channeld->te_trailers = grpc_mdelem_from_strings(mdctx, "te", "trailers");
-  channeld->status_md = grpc_mdelem_from_strings(mdctx, ":status", "200");
+  channeld->status = grpc_mdelem_from_strings(mdctx, ":status", "200");
+  channeld->method = grpc_mdelem_from_strings(mdctx, ":method", "POST");
+  channeld->http_scheme = grpc_mdelem_from_strings(mdctx, ":scheme", "http");
+  channeld->https_scheme = grpc_mdelem_from_strings(mdctx, ":scheme", "https");
+  channeld->grpc_scheme = grpc_mdelem_from_strings(mdctx, ":scheme", "grpc");
+  channeld->content_type =
+      grpc_mdelem_from_strings(mdctx, "content-type", "application/grpc");
 }
 
 /* Destructor for channel data */
@@ -160,7 +235,12 @@
   channel_data *channeld = elem->channel_data;
 
   grpc_mdelem_unref(channeld->te_trailers);
-  grpc_mdelem_unref(channeld->status_md);
+  grpc_mdelem_unref(channeld->status);
+  grpc_mdelem_unref(channeld->method);
+  grpc_mdelem_unref(channeld->http_scheme);
+  grpc_mdelem_unref(channeld->https_scheme);
+  grpc_mdelem_unref(channeld->grpc_scheme);
+  grpc_mdelem_unref(channeld->content_type);
 }
 
 const grpc_channel_filter grpc_http_server_filter = {