Add a new method ParseQuicVersionLabelString() for parsing string-ified QUIC version labels (but not other formats). PiperOrigin-RevId: 427339522

diff --git a/quic/core/quic_versions.cc b/quic/core/quic_versions.cc
index c457dbf..9dd0cc9 100644
--- a/quic/core/quic_versions.cc
+++ b/quic/core/quic_versions.cc

@@ -457,6 +457,18 @@
   return QuicTagToString(quiche::QuicheEndian::HostToNet32(version_label));
 }
 
+ParsedQuicVersion ParseQuicVersionLabelString(
+    absl::string_view version_label_string) {
+  const ParsedQuicVersionVector supported_versions = AllSupportedVersions();
+  for (const ParsedQuicVersion& version : supported_versions) {
+    if (version_label_string ==
+        QuicVersionLabelToString(CreateQuicVersionLabel(version))) {
+      return version;
+    }
+  }
+  return UnsupportedQuicVersion();
+}
+
 std::string QuicVersionLabelVectorToString(
     const QuicVersionLabelVector& version_labels, const std::string& separator,
     size_t skip_after_nth_version) {

diff --git a/quic/core/quic_versions.h b/quic/core/quic_versions.h
index c6e2406..4e16fef 100644
--- a/quic/core/quic_versions.h
+++ b/quic/core/quic_versions.h

@@ -490,6 +490,15 @@
 QUIC_EXPORT_PRIVATE std::string QuicVersionLabelToString(
     QuicVersionLabel version_label);
 
+// Helper function which translates from a QuicVersionLabel string to a
+// ParsedQuicVersion. The version label string must be of the form returned
+// by QuicVersionLabelToString, for example, "00000001" or "Q046", but not
+// "51303433" (the hex encoding of the Q064 version label). Returns
+// the ParsedQuicVersion which matches the label or UnsupportedQuicVersion()
+// otherwise.
+QUIC_EXPORT_PRIVATE ParsedQuicVersion
+ParseQuicVersionLabelString(absl::string_view version_label_string);
+
 // Returns |separator|-separated list of string representations of
 // QuicVersionLabel values in the supplied |version_labels| vector. The values
 // after the (0-based) |skip_after_nth_version|'th are skipped.

diff --git a/quic/core/quic_versions_test.cc b/quic/core/quic_versions_test.cc
index 5e8e97b..4dcad1c 100644
--- a/quic/core/quic_versions_test.cc
+++ b/quic/core/quic_versions_test.cc

@@ -274,6 +274,34 @@
   EXPECT_EQ("Q035,T038,ff000007", os.str());
 }
 
+TEST(QuicVersionsTest, ParseQuicVersionLabelString) {
+  static_assert(SupportedVersions().size() == 5u,
+                "Supported versions out of sync");
+  // Explicitly test known QUIC version label strings.
+  EXPECT_EQ(ParsedQuicVersion::Q043(), ParseQuicVersionLabelString("Q043"));
+  EXPECT_EQ(ParsedQuicVersion::Q046(), ParseQuicVersionLabelString("Q046"));
+  EXPECT_EQ(ParsedQuicVersion::Q050(), ParseQuicVersionLabelString("Q050"));
+  EXPECT_EQ(ParsedQuicVersion::Draft29(),
+            ParseQuicVersionLabelString("ff00001d"));
+  EXPECT_EQ(ParsedQuicVersion::RFCv1(),
+            ParseQuicVersionLabelString("00000001"));
+
+  // Sanity check that a variety of other serialization formats are ignored.
+  EXPECT_EQ(UnsupportedQuicVersion(), ParseQuicVersionLabelString("1"));
+  EXPECT_EQ(UnsupportedQuicVersion(), ParseQuicVersionLabelString("46"));
+  EXPECT_EQ(UnsupportedQuicVersion(),
+            ParseQuicVersionLabelString("QUIC_VERSION_46"));
+  EXPECT_EQ(UnsupportedQuicVersion(), ParseQuicVersionLabelString("h3"));
+  EXPECT_EQ(UnsupportedQuicVersion(), ParseQuicVersionLabelString("h3-29"));
+
+  // Test round-trips between QuicVersionLabelToString and
+  // ParseQuicVersionLabelString.
+  for (const ParsedQuicVersion& version : AllSupportedVersions()) {
+    EXPECT_EQ(version, ParseQuicVersionLabelString(QuicVersionLabelToString(
+                           CreateQuicVersionLabel(version))));
+  }
+}
+
 TEST(QuicVersionsTest, QuicVersionToString) {
   EXPECT_EQ("QUIC_VERSION_UNSUPPORTED",
             QuicVersionToString(QUIC_VERSION_UNSUPPORTED));