Don't panic encoding arrays where the first item is a table, but others aren't

It would try to encode this:

	arr = [{}, 0]

As:

	[[arr]]

	[[arr]]

Because it would only check the type of the first array entry. This made
sense before TOML 1.0 since all array entries had to be the same type,
but now we need to check all elements.

Author: arp242

.gitignore

@@ -1,2 +1,2 @@
-toml.test
+/toml.test
 /toml-test

encode.go

@@ -551,7 +551,7 @@ func tomlTypeOfGo(rv reflect.Value) tomlType {
 	case reflect.Float32, reflect.Float64:
 		return tomlFloat
 	case reflect.Array, reflect.Slice:
-		if typeEqual(tomlHash, tomlArrayType(rv)) {
+		if isTableArray(rv) {
 			return tomlArrayHash
 		}
 		return tomlArray
@@ -599,29 +599,25 @@ func isMarshaler(rv reflect.Value) bool {
 	return false
 }
 
-// tomlArrayType returns the element type of a TOML array. The type returned
-// may be nil if it cannot be determined (e.g., a nil slice or a zero length
-// slize). This function may also panic if it finds a type that cannot be
-// expressed in TOML (such as nil elements, heterogeneous arrays or directly
-// nested arrays of tables).
-func tomlArrayType(rv reflect.Value) tomlType {
-	if isNil(rv) || !rv.IsValid() || rv.Len() == 0 {
-		return nil
+// isTableArray reports if all entries in the array or slice are a table.
+func isTableArray(arr reflect.Value) bool {
+	if isNil(arr) || !arr.IsValid() || arr.Len() == 0 {
+		return false
 	}
 
 	/// Don't allow nil.
-	rvlen := rv.Len()
-	for i := 1; i < rvlen; i++ {
-		if tomlTypeOfGo(rv.Index(i)) == nil {
+	for i := 0; i < arr.Len(); i++ {
+		if tomlTypeOfGo(arr.Index(i)) == nil {
 			encPanic(errArrayNilElement)
 		}
 	}
 
-	firstType := tomlTypeOfGo(rv.Index(0))
-	if firstType == nil {
-		encPanic(errArrayNilElement)
+	for i := 0; i < arr.Len(); i++ {
+		if !typeEqual(tomlHash, tomlTypeOfGo(arr.Index(i))) {
+			return false
+		}
 	}
-	return firstType
+	return true
 }
 
 type tagOptions struct {

fuzz_test.go

@@ -12,29 +12,59 @@ func FuzzDecode(f *testing.F) {
 	buf := make([]byte, 0, 2048)
 
 	f.Add(`
-# This is a TOML document
+# This is an example TOML document which shows most of its features.
 
-title = "TOML Example"
+# Simple key/value with a string.
+title = "TOML example \U0001F60A"
 
-[owner]
-name = "Tom Preston-Werner"
-dob = 1979-05-27T07:32:00-08:00
+desc = """
+An example TOML document. \
+"""
 
-[database]
-enabled = true
-ports = [ 8000, 8001, 8002 ]
-data = [ ["delta", "phi"], [3.14] ]
-temp_targets = { cpu = 79.5, case = 72.0 }
+# Array with integers and floats in the various allowed formats.
+integers = [42, 0x42, 0o42, 0b0110]
+floats   = [1.42, 1e-02]
 
-[servers]
+# Array with supported datetime formats.
+times = [
+	2021-11-09T15:16:17+01:00,  # datetime with timezone.
+	2021-11-09T15:16:17Z,       # UTC datetime.
+	2021-11-09T15:16:17,        # local datetime.
+	2021-11-09,                 # local date.
+	15:16:17,                   # local time.
+]
 
-[servers.alpha]
-ip = "10.0.0.1"
-role = "frontend"
+# Durations.
+duration = ["4m49s", "8m03s", "1231h15m55s"]
+
+# Table with inline tables.
+distros = [
+	{name = "Arch Linux", packages = "pacman"},
+	{name = "Void Linux", packages = "xbps"},
+	{name = "Debian",     packages = "apt"},
+]
 
+# Create new table; note the "servers" table is created implicitly.
+[servers.alpha]
+	# You can indent as you please, tabs or spaces.
+	ip        = '10.0.0.1'
+	hostname  = 'server1'
+	enabled   = false
 [servers.beta]
-ip = "10.0.0.2"
-role = "backend"
+	ip        = '10.0.0.2'
+	hostname  = 'server2'
+	enabled   = true
+
+# Start a new table array; note that the "characters" table is created implicitly.
+[[characters.star-trek]]
+	name = "James Kirk"
+	rank = "Captain\u0012 \t"
+[[characters.star-trek]]
+	name = "Spock"
+	rank = "Science officer"
+
+[undecoded] # To show the MetaData.Undecoded() feature.
+	key = "This table intentionally left undecoded"
 `)
 	f.Fuzz(func(t *testing.T, file string) {
 		var m map[string]interface{}

internal/toml-test/tests/valid/array/mixed-string-table.json

@@ -18,5 +18,21 @@
         "value": "https://example.com/bazqux"
       }
     }
+  ],
+  "mixed": [
+    {
+      "k": {
+        "type": "string",
+        "value": "a"
+      }
+    },
+    {
+      "type": "string",
+      "value": "b"
+    },
+    {
+      "type": "integer",
+      "value": "1"
+    }
   ]
 }

internal/toml-test/tests/valid/array/mixed-string-table.toml

@@ -2,3 +2,10 @@ contributors = [
   "Foo Bar <[email protected]>",
   { name = "Baz Qux", email = "[email protected]", url = "https://example.com/bazqux" }
 ]
+
+# Start with a table as the first element. This tests a case that some libraries
+# might have where they will check if the first entry is a table/map/hash/assoc
+# array and then encode it as a table array. This was a reasonable thing to do
+# before TOML 1.0 since arrays could only contain one type, but now it's no
+# longer.
+mixed = [{k="a"}, "b", 1]