[Rolling release] Do not forward legacy cmd.exe variables to the se…

…rver (bazelbuild#24500)

This fixes startup failures such as:
```
ERROR: While parsing option --client_env==C:=C:\Users\wyv: Variable definitions must be in the form of a 'name=value' assignment
```

Fixes bazelbuild#24448

Closes bazelbuild#24451.

PiperOrigin-RevId: 699115724
Change-Id: Id7a2445e82b24384e9ade0c92ddb343f00e9ab2b

Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>

src/main/cpp/option_processor_windows.cc

@@ -63,7 +63,14 @@ static void PreprocessEnvString(std::string* env_str) {
 #endif  // defined(__CYGWIN__)
 
 static bool IsValidEnvName(std::string_view s) {
-  std::string_view name = s.substr(0, s.find('='));
+  std::size_t first_equal = s.find('=');
+  if (first_equal == 0) {
+    // Skip over legacy environment variables that start with '=', e.g. '=C:'
+    // These are set by cmd.exe and can't be parsed by the Bazel server if
+    // passed into --client_env.
+    return false;
+  }
+  std::string_view name = s.substr(0, first_equal);
   return std::all_of(name.begin(), name.end(), [](char c) {
     return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') ||
            (c >= '0' && c <= '9') || c == '_' || c == '(' || c == ')';

src/test/shell/integration/run_test.sh

@@ -678,47 +678,6 @@ EOF
   fi
 }
 
-function test_run_under_command_change_preserves_cache() {
-  if $is_windows; then
-    echo "This test requires --run_under to be able to run echo."
-    return
-  fi
-
-  local -r pkg="pkg${LINENO}"
-  mkdir -p "${pkg}"
-  cat > "$pkg/BUILD" <<'EOF'
-load(":defs.bzl", "my_rule")
-my_rule(
-  name = "my_rule",
-)
-EOF
-  cat > "$pkg/defs.bzl" <<'EOF'
-def _my_rule_impl(ctx):
-  print("my_rule is being analyzed")
-  out = ctx.actions.declare_file(ctx.label.name)
-  ctx.actions.write(out, "echo 'from rule'", is_executable = True)
-  return [DefaultInfo(executable = out)]
-
-my_rule = rule(
-  implementation = _my_rule_impl,
-  executable = True,
-)
-EOF
-
-  bazel run "${pkg}:my_rule" >$TEST_log 2>&1 \
-   || fail "expected run to pass"
-  expect_log "my_rule is being analyzed"
-  expect_log "from rule"
-  expect_not_log "from run_under"
-
-  # Use > to clear the previous log.
-  bazel run --run_under="echo 'from run_under' &&" "${pkg}:my_rule" >$TEST_log 2>&1 \
-   || fail "expected run to pass"
-  expect_not_log "my_rule is being analyzed"
-  expect_log "from rule"
-  expect_log "from run_under"
-}
-
 function test_build_id_env_var() {
   local -r pkg="pkg${LINENO}"
   mkdir -p "${pkg}"