Merge pull request #2 from skx/fuzzing

This pull-request adds fuzz-testing, with the standard golang tools, and resolves many many issues that were discovered by that.

README.md

@@ -99,7 +99,7 @@ We have a reasonable number of functions implemented in our golang core:
 * Comparison functions:
   * `<`, `<=`, `>`, `>=`, `=`, & `eq`.
 * Misc features
-  * `str`, `print`, & `type`
+  * `error`, `str`, `print`, & `type`
 * Special forms
   * `begin`, `cond`, `define`, `eval`, `if`, `lambda`, `let`,  `set!`, `quote`,
 * Tail recursion optimization.
@@ -126,6 +126,37 @@ Notable omissions here:
 * No vectors/hashes/records.
 
 
+## Fuzz Testing
+
+If you're working with go 1.18+ you'll be able to run a fuzz-test of the interpreter, without the need for any external tools.
+
+Run the included driver like so:
+
+```sh
+go test -fuzztime=300s -parallel=1 -fuzz=FuzzYAL -v
+```
+
+Sample output will look like this:
+
+```
+=== FUZZ  FuzzYAL
+fuzz: elapsed: 0s, gathering baseline coverage: 0/38 completed
+fuzz: elapsed: 0s, gathering baseline coverage: 38/38 completed, now fuzzing with 1 workers
+fuzz: elapsed: 3s, execs: 39 (13/sec), new interesting: 0 (total: 38)
+fuzz: elapsed: 6s, execs: 39 (0/sec), new interesting: 0 (total: 38)
+fuzz: elapsed: 9s, execs: 39 (0/sec), new interesting: 0 (total: 38)
+fuzz: elapsed: 12s, execs: 39 (0/sec), new interesting: 0 (total: 38)
+fuzz: elapsed: 15s, execs: 39 (0/sec), new interesting: 0 (total: 38)
+fuzz: elapsed: 18s, execs: 39 (0/sec), new interesting: 0 (total: 38)
+fuzz: elapsed: 21s, execs: 39 (0/sec), new interesting: 0 (total: 38)
+fuzz: elapsed: 24s, execs: 39 (0/sec), new interesting: 0 (total: 38)
+fuzz: elapsed: 27s, execs: 39 (0/sec), new interesting: 0 (total: 38)
+```
+
+If you find a crash then it is either a bug which needs to be fixed, or a false-positive (i.e. a function reports an error which is expected) in which case the fuzz-test should be updated to add it to the list of known-OK results.  (For example "division by zero" is a fatal error, so that's a known-OK result).
+
+
+
 
 ## References
 

builtins/builtins.go

@@ -239,7 +239,13 @@ func modFn(args []primitive.Primitive) primitive.Primitive {
 	if _, ok := args[1].(primitive.Number); !ok {
 		return primitive.Error("argument not a number")
 	}
-	return primitive.Number(int(args[0].(primitive.Number)) % int(args[1].(primitive.Number)))
+
+	a := int(args[0].(primitive.Number))
+	b := int(args[1].(primitive.Number))
+	if b == 0 {
+		return primitive.Error("attempted division by zero")
+	}
+	return primitive.Number(a % b)
 }
 
 // expnFn implements "#"
@@ -306,7 +312,14 @@ func carFn(args []primitive.Primitive) primitive.Primitive {
 		return primitive.Error("argument not a list")
 	}
 
-	return args[0].(primitive.List)[0]
+	// If we have at least one entry then return the first
+	lst := args[0].(primitive.List)
+	if len(lst) > 0 {
+		return lst[0]
+	}
+
+	// Otherwise return nil
+	return primitive.Nil{}
 }
 
 // cdrFn implements "cdr"
@@ -321,7 +334,11 @@ func cdrFn(args []primitive.Primitive) primitive.Primitive {
 		return primitive.Error("argument not a list")
 	}
 
-	return args[0].(primitive.List)[1:]
+	lst := args[0].(primitive.List)
+	if len(lst) > 0 {
+		return lst[1:]
+	}
+	return primitive.Nil{}
 }
 
 // errorFn implements "error"

builtins/builtins_test.go

@@ -364,6 +364,23 @@ func TestMod(t *testing.T) {
 		t.Fatalf("got error, but wrong one %v", out)
 	}
 
+	//
+	// Mod 0
+	//
+	out = modFn([]primitive.Primitive{
+		primitive.Number(32),
+		primitive.Number(0),
+	})
+
+	// Will lead to an error
+	e, ok = out.(primitive.Error)
+	if !ok {
+		t.Fatalf("expected error, got %v", out)
+	}
+	if !strings.Contains(string(e), "division by zero") {
+		t.Fatalf("got error, but wrong one %v", out)
+	}
+
 	//
 	// Now a real one
 	//
@@ -650,6 +667,18 @@ func TestCar(t *testing.T) {
 	if r.ToString() != "3" {
 		t.Fatalf("got wrong result : %v", r)
 	}
+
+	// Now a list which is empty
+	out = carFn([]primitive.Primitive{
+		primitive.List{},
+	})
+
+	// No error
+	_, ok3 := out.(primitive.Nil)
+	if !ok3 {
+		t.Fatalf("expected nil, got %v", out)
+	}
+
 }
 
 // Test (cdr
@@ -698,6 +727,17 @@ func TestCdr(t *testing.T) {
 	if r.ToString() != "(4 5)" {
 		t.Fatalf("got wrong result : %v", r)
 	}
+
+	// Now a list which is empty
+	out = cdrFn([]primitive.Primitive{
+		primitive.List{},
+	})
+
+	// No error
+	_, ok3 := out.(primitive.Nil)
+	if !ok3 {
+		t.Fatalf("expected nil, got %v", out)
+	}
 }
 
 func TestStr(t *testing.T) {