Make PrepareEnvironmentForJvm() explicitly inherit environment variab…

…les.

The map returned by PrepareEnvironmentForJvm() previously represented a
"delta" to apply to the current environment.  This was used to modify the
environment surrounding fork/execv calls because we could not pass an envp
to execv.

However, when using the resulting map to create the envp value of a call
like posix_spawn, we must be exhaustive and list all variables we expect
the subprocess to have.  And as we are going to be using posix_spawn, we
need to make this happen, so change PrepareEnvironmentForJvm() to make
the returned map exhaustive.

Prerequisite change to address
bazelbuild#7446 because of the need to
switch to posix_spawn.

RELNOTES: None.
PiperOrigin-RevId: 234971246

src/main/cpp/blaze.cc

@@ -75,6 +75,8 @@
 
 using blaze_util::GetLastErrorString;
 
+extern char** environ;
+
 namespace blaze {
 
 using std::map;
@@ -1416,6 +1418,26 @@ static void ComputeBaseDirectories(const WorkspaceLayout *workspace_layout,
 static map<string, EnvVarValue> PrepareEnvironmentForJvm() {
   map<string, EnvVarValue> result;
 
+  // Make sure all existing environment variables appear as part of the
+  // resulting map unless they are overridden below by UNSET values.
+  //
+  // Even though the map we return is intended to represent a "delta" of
+  // environment variables to modify the current process, we may actually use
+  // such map to configure a process from scratch (via interfaces like execvpe
+  // or posix_spawn), so we need to inherit any untouched variables.
+  for (char** entry = environ; *entry != NULL; entry++) {
+    const std::string var_value = *entry;
+    std::string::size_type equals = var_value.find('=');
+    if (equals == std::string::npos) {
+      // Ignore possibly-bad environment. We don't control what we see in this
+      // global variable, so it could be invalid.
+      continue;
+    }
+    const std::string var = var_value.substr(0, equals);
+    const std::string value = var_value.substr(equals + 1);
+    result[var] = EnvVarValue(EnvVarAction::SET, value);
+  }
+
   if (!blaze::GetEnv("LD_ASSUME_KERNEL").empty()) {
     // Fix for bug: if ulimit -s and LD_ASSUME_KERNEL are both
     // specified, the JVM fails to create threads.  See thread_stack_regtest.