Add support for ENVIRON special array

The difference between the name-value pair slice and the "key=value"
format is unfortunate, but I think consistency with config.Vars is
important, and also I've never liked os.Environ()'s format because you
usually have to strings.Split it anyway. Besides, in most cases the
default will be fine.

Fixes #73

interp/interp.go

@@ -194,6 +194,11 @@ type Config struct {
 	// Exec args used to run system shell. Typically, this will
 	// be {"/bin/sh", "-c"}
 	ShellCommand []string
+
+	// List of name-value pairs to be assigned to the ENVIRON special
+	// array, for example []string{"USER", "bob", "HOME", "/home/bob"}.
+	// If nil (the default), values from os.Environ() are used.
+	Environ []string
 }
 
 // ExecProgram executes the parsed program using the given interpreter
@@ -204,6 +209,9 @@ func ExecProgram(program *Program, config *Config) (int, error) {
 	if len(config.Vars)%2 != 0 {
 		return 0, newError("length of config.Vars must be a multiple of 2, not %d", len(config.Vars))
 	}
+	if len(config.Environ)%2 != 0 {
+		return 0, newError("length of config.Environ must be a multiple of 2, not %d", len(config.Environ))
+	}
 
 	p := &interp{program: program}
 
@@ -252,6 +260,21 @@ func ExecProgram(program *Program, config *Config) (int, error) {
 		}
 	}
 
+	// Setup ENVIRON from config or environment variables
+	environIndex := program.Arrays["ENVIRON"]
+	if config.Environ != nil {
+		for i := 0; i < len(config.Environ); i += 2 {
+			p.setArrayValue(ScopeGlobal, environIndex, config.Environ[i], numStr(config.Environ[i+1]))
+		}
+	} else {
+		for _, kv := range os.Environ() {
+			eq := strings.IndexByte(kv, '=')
+			if eq >= 0 {
+				p.setArrayValue(ScopeGlobal, environIndex, kv[:eq], numStr(kv[eq+1:]))
+			}
+		}
+	}
+
 	// Setup system shell command
 	if len(config.ShellCommand) != 0 {
 		p.shellCommand = config.ShellCommand

interp/interp_test.go

@@ -1114,6 +1114,33 @@ func TestFlushError(t *testing.T) {
 	}
 }
 
+func TestEnviron(t *testing.T) {
+	os.Setenv("GOAWK_TEN", "10") // to test that ENVIRON[x] is numeric string
+	src := `
+BEGIN {
+	n = 0
+	for (k in ENVIRON)
+		n++
+	print(n, ENVIRON["USER"], ENVIRON["GOAWK_TEN"] < 2)
+}`
+	expected := fmt.Sprintf("%d %s 0\n", len(os.Environ()), os.Getenv("USER"))
+	testGoAWK(t, src, "", expected, "", nil, nil)
+
+	expected = "2 bob 0\n"
+	testGoAWK(t, src, "", expected, "", nil, func(config *interp.Config) {
+		config.Environ = []string{"USER", "bob", "GOAWK_TEN", "10"}
+	})
+
+	expected = "0  1\n"
+	testGoAWK(t, src, "", expected, "", nil, func(config *interp.Config) {
+		config.Environ = []string{}
+	})
+
+	testGoAWK(t, src, "", "", "length of config.Environ must be a multiple of 2, not 3", nil, func(config *interp.Config) {
+		config.Environ = []string{"b", "a", "d"}
+	})
+}
+
 func benchmarkProgram(b *testing.B, funcs map[string]interface{},
 	input, expected, srcFormat string, args ...interface{},
 ) {

parser/resolve.go

@@ -75,7 +75,8 @@ func (p *parser) initResolve() {
 	p.varTypes = make(map[string]map[string]typeInfo)
 	p.varTypes[""] = make(map[string]typeInfo) // globals
 	p.functions = make(map[string]int)
-	p.arrayRef("ARGV", Position{1, 1}) // interpreter relies on ARGV being present
+	p.arrayRef("ARGV", Position{1, 1})    // interpreter relies on ARGV being present
+	p.arrayRef("ENVIRON", Position{1, 1}) // and ENVIRON
 	p.multiExprs = make(map[*MultiExpr]Position, 3)
 }