Update bazel's emacs check for 25.1's INSIDE_EMACS move.

History in
https://github.com/emacs-mirror/emacs/blo[]f125aa3de06fa0180a83ec7b5a26970309eeeb6/etc/NEWS#L1769-L1773

RELNOTES: Emacs' [C-x `], a.k.a. next-error, works again in emacsen >= 25.1

--
MOS_MIGRATED_REVID=131851164

src/main/cpp/blaze_util.cc

@@ -224,15 +224,25 @@ bool UnlinkPath(const string &file_path) {
   return unlink(file_path.c_str()) == 0;
 }
 
+bool IsEmacsTerminal() {
+  string emacs = getenv("EMACS") == nullptr ? "" : getenv("EMACS");
+  string inside_emacs =
+      getenv("INSIDE_EMACS") == nullptr ? "" : getenv("INSIDE_EMACS");
+  // GNU Emacs <25.1 (and ~all non-GNU emacsen) set EMACS=t, but >=25.1 doesn't
+  // do that and instead sets INSIDE_EMACS=<stuff> (where <stuff> can look like
+  // e.g. "25.1.1,comint").  So we check both variables for maximum
+  // compatibility.
+  return emacs == "t" || inside_emacs != "";
+}
+
 // Returns true iff both stdout and stderr are connected to a
 // terminal, and it can support color and cursor movement
 // (this is computed heuristically based on the values of
 // environment variables).
 bool IsStandardTerminal() {
   string term = getenv("TERM") == nullptr ? "" : getenv("TERM");
-  string emacs = getenv("EMACS") == nullptr ? "" : getenv("EMACS");
   if (term == "" || term == "dumb" || term == "emacs" || term == "xterm-mono" ||
-      term == "symbolics" || term == "9term" || emacs == "t") {
+      term == "symbolics" || term == "9term" || IsEmacsTerminal()) {
     return false;
   }
   return isatty(STDOUT_FILENO) && isatty(STDERR_FILENO);

src/main/cpp/blaze_util.h

@@ -64,6 +64,9 @@ bool WriteFile(const string &content, const string &filename);
 // Returns true on success. In case of failure sets errno.
 bool UnlinkPath(const string &file_path);
 
+// Returns true iff the current terminal is running inside an Emacs.
+bool IsEmacsTerminal();
+
 // Returns true iff the current terminal can support color and cursor movement.
 bool IsStandardTerminal();
 

src/main/cpp/option_processor.cc

@@ -443,8 +443,7 @@ void OptionProcessor::AddRcfileArgsAndOptions(bool batch, const string& cwd) {
   }
   command_arguments_.push_back("--client_cwd=" + blaze::ConvertPath(cwd));
 
-  const char *emacs = getenv("EMACS");
-  if (emacs != NULL && strcmp(emacs, "t") == 0) {
+  if (IsEmacsTerminal()) {
     command_arguments_.push_back("--emacs");
   }
 }

src/main/java/com/google/devtools/build/lib/runtime/BlazeCommandEventHandler.java

@@ -112,8 +112,9 @@ public static class Options extends OptionsBase {
     @Option(name = "emacs",
             defaultValue = "false",
             category = "undocumented",
-            help = "A system-generated parameter which is true iff EMACS=t in the environment of "
-               + "the client.  This option controls certain display features.")
+            help = "A system-generated parameter which is true iff EMACS=t or INSIDE_EMACS is set "
+               + "in the environment of the client.  This option controls certain display "
+               + "features.")
     public boolean runningInEmacs;
 
     @Option(name = "show_timestamps",