6924920: Class Data Sharing limit on the java version string can create failures

Summary: Truncate the java version string and add a hash value if it is too long.
Reviewed-by: dholmes, coleenp

src/share/vm/memory/filemap.cpp

@@ -25,6 +25,7 @@
 #include "precompiled.hpp"
 #include "classfile/classLoader.hpp"
 #include "classfile/symbolTable.hpp"
+#include "classfile/altHashing.hpp"
 #include "memory/filemap.hpp"
 #include "runtime/arguments.hpp"
 #include "runtime/java.hpp"
@@ -82,9 +83,38 @@ void FileMapInfo::fail_continue(const char *msg, ...) {
   close();
 }
 
-
 // Fill in the fileMapInfo structure with data about this VM instance.
 
+// This method copies the vm version info into header_version.  If the version is too
+// long then a truncated version, which has a hash code appended to it, is copied.
+//
+// Using a template enables this method to verify that header_version is an array of
+// length JVM_IDENT_MAX.  This ensures that the code that writes to the CDS file and
+// the code that reads the CDS file will both use the same size buffer.  Hence, will
+// use identical truncation.  This is necessary for matching of truncated versions.
+template <int N> static void get_header_version(char (&header_version) [N]) {
+  assert(N == JVM_IDENT_MAX, "Bad header_version size");
+
+  const char *vm_version = VM_Version::internal_vm_info_string();
+  const int version_len = (int)strlen(vm_version);
+
+  if (version_len < (JVM_IDENT_MAX-1)) {
+    strcpy(header_version, vm_version);
+
+  } else {
+    // Get the hash value.  Use a static seed because the hash needs to return the same
+    // value over multiple jvm invocations.
+    unsigned int hash = AltHashing::murmur3_32(8191, (const jbyte*)vm_version, version_len);
+
+    // Truncate the ident, saving room for the 8 hex character hash value.
+    strncpy(header_version, vm_version, JVM_IDENT_MAX-9);
+
+    // Append the hash code as eight hex digits.
+    sprintf(&header_version[JVM_IDENT_MAX-9], "%08x", hash);
+    header_version[JVM_IDENT_MAX-1] = 0;  // Null terminate.
+  }
+}
+
 void FileMapInfo::populate_header(size_t alignment) {
   _header._magic = 0xf00baba2;
   _header._version = _current_version;
@@ -95,13 +125,7 @@ void FileMapInfo::populate_header(size_t alignment) {
   // invoked with.
 
   // JVM version string ... changes on each build.
-  const char *vm_version = VM_Version::internal_vm_info_string();
-  if (strlen(vm_version) < (JVM_IDENT_MAX-1)) {
-    strcpy(_header._jvm_ident, vm_version);
-  } else {
-    fail_stop("JVM Ident field for shared archive is too long"
-              " - truncated to <%s>", _header._jvm_ident);
-  }
+  get_header_version(_header._jvm_ident);
 
   // Build checks on classpath and jar files
   _header._num_jars = 0;
@@ -434,8 +458,9 @@ bool FileMapInfo::validate() {
     fail_continue("The shared archive file has a bad magic number.");
     return false;
   }
-  if (strncmp(_header._jvm_ident, VM_Version::internal_vm_info_string(),
-              JVM_IDENT_MAX-1) != 0) {
+  char header_version[JVM_IDENT_MAX];
+  get_header_version(header_version);
+  if (strncmp(_header._jvm_ident, header_version, JVM_IDENT_MAX-1) != 0) {
     fail_continue("The shared archive file was created by a different"
                   " version or build of HotSpot.");
     return false;