reenable pipelining via hashsum reordering support

Now that methods have the expected hashes available they can check if
the response from the server is what they expected. Pipelining is one of
those areas in which servers can mess up by not supporting it properly,
which forced us to disable it for the time being. Now, we check if
we got a response out of order, which we can not only use to disable
pipelining automatically for the next requests, but we can fix it up
just like the server responded in proper order for the current requests.

To ensure that this little trick works pipelining is only attempt if we
have hashsums for all the files in the chain which in theory reduces the
use of pipelining usage even on the many servers which work properly,
but in practice only the InRelease file (or similar such) will be
requested without a hashsum – and as it is the only file requested in
that stage it can't be pipelined even if we wanted to.

Some minor annoyances remain: The display of the progress we have
doesn't reflect this change, so it looks like the same package gets
downloaded multiple times while others aren't at all. Further more,
partial files are not supported in this recovery as the received data
was appended to the wrong file, so the hashsum doesn't match.

Both seem to be minor enough to reenable pipelining by default until
further notice through to test if it really solves the problem.
This therefore reverts commit 8221431757c775ee875a061b184b5f6f2330f928.

2 files changed, 43 insertions, 5 deletions

diff --git a/methods/server.cc b/methods/server.cc
index 5a13f18a7..c91d3b218 100644
--- a/methods/server.cc
+++ b/methods/server.cc
@@ -392,9 +392,16 @@ bool ServerMethod::Fetch(FetchItem *)
    for (FetchItem *I = Queue; I != 0 && Depth < (signed)PipelineDepth; 
 	I = I->Next, Depth++)
    {
-      // If pipelining is disabled, we only queue 1 request
-      if (Server->Pipeline == false && Depth >= 0)
-	 break;
+      if (Depth >= 0)
+      {
+	 // If pipelining is disabled, we only queue 1 request
+	 if (Server->Pipeline == false)
+	    break;
+	 // if we have no hashes, do at most one such request
+	 // as we can't fixup pipeling misbehaviors otherwise
+	 else if (I->ExpectedHashes.usable() == false)
+	    break;
+      }
       
       // Make sure we stick with the same server
       if (Server->Comp(I->Uri) == false)
@@ -546,7 +553,38 @@ int ServerMethod::Loop()
 	    // Send status to APT
 	    if (Result == true)
 	    {
-	       Res.TakeHashes(*Server->GetHashes());
+	       Hashes * const resultHashes = Server->GetHashes();
+	       HashStringList const hashList = resultHashes->GetHashStringList();
+	       if (PipelineDepth != 0 && Queue->ExpectedHashes.usable() == true && Queue->ExpectedHashes != hashList)
+	       {
+		  // we did not get the expected hash… mhhh:
+		  // could it be that server/proxy messed up pipelining?
+		  FetchItem * BeforeI = Queue;
+		  for (FetchItem *I = Queue->Next; I != 0 && I != QueueBack; I = I->Next)
+		  {
+		     if (I->ExpectedHashes.usable() == true && I->ExpectedHashes == hashList)
+		     {
+			// yes, he did! Disable pipelining and rewrite queue
+			if (Server->Pipeline == true)
+			{
+			   // FIXME: fake a warning message as we have no proper way of communicating here
+			   std::string out;
+			   strprintf(out, _("Automatically disabled %s due to incorrect response from server/proxy. (man 5 apt.conf)"), "Acquire::http::PipelineDepth");
+			   std::cerr << "W: " << out << std::endl;
+			   Server->Pipeline = false;
+			   // we keep the PipelineDepth value so that the rest of the queue can be fixed up as well
+			}
+			Rename(Res.Filename, I->DestFile);
+			Res.Filename = I->DestFile;
+			BeforeI->Next = I->Next;
+			I->Next = Queue;
+			Queue = I;
+			break;
+		     }
+		     BeforeI = I;
+		  }
+	       }
+	       Res.TakeHashes(*resultHashes);
 	       URIDone(Res);
 	    }
 	    else
diff --git a/methods/server.h b/methods/server.h
index 0f45ab994..5299b3954 100644
--- a/methods/server.h
+++ b/methods/server.h
@@ -140,7 +140,7 @@ class ServerMethod : public pkgAcqMethod
    virtual ServerState * CreateServerState(URI uri) = 0;
    virtual void RotateDNS() = 0;
 
-   ServerMethod(const char *Ver,unsigned long Flags = 0) : pkgAcqMethod(Ver, Flags), Server(NULL), File(NULL), PipelineDepth(0), AllowRedirect(false), Debug(false) {};
+   ServerMethod(const char *Ver,unsigned long Flags = 0) : pkgAcqMethod(Ver, Flags), Server(NULL), File(NULL), PipelineDepth(10), AllowRedirect(false), Debug(false) {};
    virtual ~ServerMethod() {};
 };