httpcaddyfile: tls: Load repeated cert files only once, with one tag

See end of issue #3004. Loading the same certificate file multiple times
with different tags will result in it being de-duplicated in the in-
memory cache, because of course they all have the same bytes. This
meant that any certs of the same filename loaded with different tags
would be overwritten by the next certificate of the same filename, and
any conn policies looking for the tags of the previous ones would never
find them, causing connections to fail.

So, now we remember cert filenames and their tags, instead of loading
them multiple times and overwriting previous ones.

A user crafting their own JSON might make this error too... maybe we
won't see it happen. But if it does, one possibility is, when loading
a duplicate cert, instead of discarding it completely, merge the tag
list into the one that's already stored in the cache, then discard.

caddyconfig/httpcaddyfile/builtins.go

@@ -132,20 +132,41 @@ func parseTLS(h Helper) ([]ConfigValue, error) {
 			}
 			mgr.Email = firstLine[0]
 		case 2:
-			tag := fmt.Sprintf("cert%d", tagCounter)
-			fileLoader = append(fileLoader, caddytls.CertKeyFilePair{
-				Certificate: firstLine[0],
-				Key:         firstLine[1],
-				Tags:        []string{tag},
-			})
+			certFilename := firstLine[0]
+			keyFilename := firstLine[1]
+
 			// tag this certificate so if multiple certs match, specifically
 			// this one that the user has provided will be used, see #2588:
-			// https://github.com/caddyserver/caddy/issues/2588
-			tagCounter++
+			// https://github.com/caddyserver/caddy/issues/2588 ... but we
+			// must be careful about how we do this; being careless will
+			// lead to failed handshakes
+
+			// we need to remember which cert files we've seen, since we
+			// must load each cert only once; otherwise, they each get a
+			// different tag... since a cert loaded twice has the same
+			// bytes, it will overwrite the first one in the cache, and
+			// only the last cert (and its tag) will survive, so a any conn
+			// policy that is looking for any tag but the last one to be
+			// loaded won't find it, and TLS handshakes will fail (see end)
+			// of issue #3004)
+			tag, ok := tlsCertTags[certFilename]
+			if !ok {
+				// haven't seen this cert file yet, let's give it a tag
+				// and add a loader for it
+				tag = fmt.Sprintf("cert%d", len(tlsCertTags))
+				fileLoader = append(fileLoader, caddytls.CertKeyFilePair{
+					Certificate: certFilename,
+					Key:         keyFilename,
+					Tags:        []string{tag},
+				})
+				// remember this for next time we see this cert file
+				tlsCertTags[certFilename] = tag
+			}
 			certSelector := caddytls.CustomCertSelectionPolicy{Tag: tag}
 			if cp == nil {
 				cp = new(caddytls.ConnectionPolicy)
 			}
+
 			cp.CertSelection = caddyconfig.JSONModuleObject(certSelector, "policy", "custom", h.warnings)
 		default:
 			return nil, h.ArgErr()
@@ -405,4 +426,9 @@ func parseHandleErrors(h Helper) ([]ConfigValue, error) {
 	}, nil
 }
 
-var tagCounter = 0
+// tlsCertTags maps certificate filenames to their tag.
+// This is used to remember which tag is used for each
+// certificate files, since we need to avoid loading
+// the same certificate files more than once, overwriting
+// previous tags
+var tlsCertTags = make(map[string]string)