acc: Do env var substitution in EnvMatrix values (#2678)

## Changes
When a $VAR reference is used in value of EnvMatrix, it'll be replaced
with its value if there is such variable previously defined.

## Why
It's useful to define variables in terms of other variables. For example
you can define "$MYCLI=$CLI --someparam". I'm using it to define CLI
command that either runs from bundle root or another directory.

## Tests
Unit tests + acceptance test.

Author: denik

acceptance/acceptance_test.go

@@ -486,8 +486,16 @@ func runTest(t *testing.T, dir, coverDir string, repls testdiff.ReplacementsCont
 	for _, keyvalue := range customEnv {
 		items := strings.SplitN(keyvalue, "=", 2)
 		require.Len(t, items, 2)
-		cmd.Env = append(cmd.Env, keyvalue)
-		repls.Set(items[1], "["+items[0]+"]")
+		key := items[0]
+		value := items[1]
+		newValue, newValueWithPlaceholders := internal.SubstituteEnv(value, cmd.Env)
+		if value != newValue {
+			t.Logf("Substituted %s %#v -> %#v (%#v)", key, value, newValue, newValueWithPlaceholders)
+		}
+		cmd.Env = append(cmd.Env, key+"="+newValue)
+		repls.Set(newValue, "["+key+"]")
+		// newValue won't match because parts of it were already replaced; we adding it anyway just in case but we need newValueWithPlaceholders:
+		repls.Set(newValueWithPlaceholders, "["+key+"]")
 	}
 
 	absDir, err := filepath.Abs(dir)

acceptance/internal/util.go

@@ -0,0 +1,23 @@
+package internal
+
+import (
+	"regexp"
+	"strings"
+)
+
+func SubstituteEnv(value string, env []string) (actual, placeholder string) {
+	actual = value
+	placeholder = value
+
+	for _, e := range env {
+		parts := strings.SplitN(e, "=", 2)
+		if len(parts) == 2 {
+			key, val := parts[0], parts[1]
+			re := regexp.MustCompile(`\$` + key + `\b`)
+			actual = re.ReplaceAllString(actual, val)
+			placeholder = re.ReplaceAllString(placeholder, "["+key+"]")
+		}
+	}
+
+	return actual, placeholder
+}

acceptance/internal/util_test.go

@@ -0,0 +1,75 @@
+package internal
+
+import (
+	"testing"
+)
+
+func TestSubstituteEnv(t *testing.T) {
+	tests := []struct {
+		name        string
+		value       string
+		env         []string
+		expected    string
+		placeholder string
+	}{
+		{
+			name:        "simple substitution",
+			value:       "$CLI",
+			env:         []string{"CLI=/bin/true"},
+			expected:    "/bin/true",
+			placeholder: "[CLI]",
+		},
+		{
+			name:        "multiple variables",
+			value:       "$HOME/$USER",
+			env:         []string{"HOME=/home", "USER=john"},
+			expected:    "/home/john",
+			placeholder: "[HOME]/[USER]",
+		},
+		{
+			name:     "no variables",
+			value:    "hello world",
+			env:      []string{"FOO=bar"},
+			expected: "hello world",
+		},
+		{
+			name:     "undefined variable",
+			value:    "$UNDEFINED",
+			env:      []string{"FOO=bar"},
+			expected: "$UNDEFINED",
+		},
+		{
+			name:        "partial substitution",
+			value:       "$FOO$BAR",
+			env:         []string{"FOO=hello"},
+			expected:    "hello$BAR",
+			placeholder: "[FOO]$BAR",
+		},
+		{
+			name:        "overlapping variable names",
+			value:       "$VAR $VARNAME",
+			env:         []string{"VAR=value", "VARNAME=longer"},
+			expected:    "value longer",
+			placeholder: "[VAR] [VARNAME]",
+		},
+		{
+			name:        "only one of overlapping variables provided",
+			value:       "$VAR $VARNAME",
+			env:         []string{"VAR=value"},
+			expected:    "value $VARNAME",
+			placeholder: "[VAR] $VARNAME",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			actual, placeholder := SubstituteEnv(tt.value, tt.env)
+			if actual != tt.expected {
+				t.Errorf("SubstituteEnv() actual = %q, want %q", actual, tt.expected)
+			}
+			if tt.placeholder != "" && placeholder != tt.placeholder {
+				t.Errorf("SubstituteEnv() placeholder = %q, want %q", placeholder, tt.placeholder)
+			}
+		})
+	}
+}

acceptance/selftest/envmatrix/inner/script

@@ -1,6 +1,5 @@
 [ "$FIRST" = "one" ] || [ "$FIRST" = "two" ]
 [ "$SECOND" = "variantA" ] || [ "$SECOND" = "variantB" ]
-[ "$THIRD" = "three" ]
 
 echo "FIRST=$FIRST"
 echo "SECOND=$SECOND"

acceptance/selftest/envmatrix/test.toml

@@ -1,3 +1,3 @@
 [EnvMatrix]
 FIRST = ["overriden-in-inner"]
-THIRD = ["three"]
+THIRD = ["three $FIRST"]