Reject --profile / -P within .mlrrc files (#358)

Per maintainer feedback on the PR: profiles are selected on the mlr
command line, not from within a .mlrrc file, so --profile / -P inside a
.mlrrc file is now a parse error -- the same way --prepipe is rejected
there -- rather than being silently ignored.

Adds a unit test, a should-fail regression case, and a docs bullet.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/src/customization.md

@@ -128,6 +128,9 @@ Semantics:
 * Since `--profile` selects a section of your `.mlrrc`, it can't be combined with `--norc`, or
   with `MLRRC=__none__` in the environment -- that's a fatal error.
 
+* Profiles are selected on the `mlr` command line, not from within a `.mlrrc` file: putting
+  `--profile` (or `-P`) inside a `.mlrrc` file is a parse error, just as `--prepipe` is.
+
 ## Where to put your .mlrrc
 
 If the environment variable `MLRRC` is set:

docs/src/customization.md.in

@@ -96,6 +96,9 @@ Semantics:
 * Since `--profile` selects a section of your `.mlrrc`, it can't be combined with `--norc`, or
   with `MLRRC=__none__` in the environment -- that's a fatal error.
 
+* Profiles are selected on the `mlr` command line, not from within a `.mlrrc` file: putting
+  `--profile` (or `-P`) inside a `.mlrrc` file is a parse error, just as `--prepipe` is.
+
 ## Where to put your .mlrrc
 
 If the environment variable `MLRRC` is set:

pkg/climain/mlrcli_mlrrc.go

@@ -231,6 +231,10 @@ func handleMlrrcLine(
 	} else if args[0] == "--load" || args[0] == "--mload" {
 		// Don't allow code execution via .mlrrc
 		return false
+	} else if args[0] == "--profile" || args[0] == "-P" {
+		// Profiles are selected on the mlr command line, not from within a
+		// .mlrrc file
+		return false
 	} else if cli.FLAG_TABLE.Parse(args, argc, &argi, options) {
 		// handled
 	} else {

pkg/climain/mlrcli_mlrrc_test.go

@@ -196,3 +196,24 @@ func TestMlrrcPrepipeStillDisallowed(t *testing.T) {
 		t.Fatal("expected error for --prepipe within a profile, got nil")
 	}
 }
+
+func TestMlrrcProfileFlagDisallowedWithinMlrrc(t *testing.T) {
+	// Profiles are selected on the mlr command line, not from within a
+	// .mlrrc file: --profile / -P there is a parse error, like --prepipe.
+	for _, line := range []string{"profile j", "--profile j", "-P j"} {
+		writeTempMlrrc(t, line+"\n")
+		options := cli.DefaultOptions()
+		err := loadMlrrc(options, "")
+		if err == nil {
+			t.Errorf("expected parse error for %q within .mlrrc, got nil", line)
+		} else if !strings.Contains(err.Error(), "parse error") {
+			t.Errorf("expected parse error for %q within .mlrrc, got: %v", line, err)
+		}
+	}
+	// Inside a selected profile section, too.
+	writeTempMlrrc(t, "[j]\nprofile p\n")
+	options := cli.DefaultOptions()
+	if err := loadMlrrc(options, "j"); err == nil {
+		t.Fatal("expected parse error for --profile within a profile section, got nil")
+	}
+}

test/cases/cli-mlrrc-profiles/0006/cmd

@@ -0,0 +1 @@
+mlr cat test/input/a.csv

test/cases/cli-mlrrc-profiles/0006/env

@@ -0,0 +1 @@
+MLRRC=test/input/mlrrc-with-profile-flag

test/cases/cli-mlrrc-profiles/0006/experr

@@ -0,0 +1 @@
+mlr: parse error at file "test/input/mlrrc-with-profile-flag" line 2: profile j

test/input/mlrrc-with-profile-flag

@@ -0,0 +1,5 @@
+icsv
+profile j
+
+[j]
+ojson