Backport of stacks: Make path.module and path.root relative, to match documentation into v1.14 (#38008)

* backport of commit 116646a

---------

Co-authored-by: Nick Fagerlund <nick.fagerlund@gmail.com>

Author: github-actions[bot]

.changes/v1.14/BUG FIXES-20251209-230000.yaml

@@ -0,0 +1,5 @@
+kind: BUG FIXES
+body: 'stacks: change absolute paths in path.module/path.root to be relative, as documented'
+time: 2025-12-09T23:00:00.316597+00:00
+custom:
+    Issue: "37982"

internal/configs/source_bundle_parser.go

@@ -67,7 +67,31 @@ func (p *SourceBundleParser) LoadConfigDir(source sourceaddrs.FinalSource) (*Mod
 		})
 		return nil, diags
 	}
-	mod.SourceDir = sourceDir
+
+	// The result of sources.LocalPathForSource is an absolute path, but we
+	// don't actually want to pass an absolute path for a module's SourceDir;
+	// doing so will cause the value of `path.module` in Terraform configs to
+	// differ across plans and applies, since tfc-agent performs plans and
+	// applies in temporary directories. Instead, we try to resolve a relative
+	// path from Terraform's working directory, which should always be a
+	// reasonable SourceDir value.
+	workDir, err := os.Getwd()
+	if err != nil {
+		diags = diags.Append(&hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Cannot resolve working directory",
+			Detail:   fmt.Sprintf("Failed to resolve current working directory: %s. This is a bug in Terraform - please report it.", err),
+		})
+	}
+	relativeSourceDir, err := filepath.Rel(workDir, sourceDir)
+	if err != nil {
+		diags = diags.Append(&hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Cannot resolve relative path",
+			Detail:   fmt.Sprintf("Failed to resolve relative path to module directory: %s. This is a bug in Terraform - please report it.", err),
+		})
+	}
+	mod.SourceDir = relativeSourceDir
 
 	return mod, diags
 }

internal/stacks/stackruntime/internal/stackeval/planning_test.go

@@ -7,7 +7,6 @@ import (
 	"context"
 	"fmt"
 	"os"
-	"path/filepath"
 	"strings"
 	"testing"
 	"time"
@@ -852,29 +851,21 @@ func TestPlanning_PathValues(t *testing.T) {
 			t.Fatalf("failed to get current working directory: %s", err)
 		}
 
-		normalizePath := func(path string) string {
-			rel, err := filepath.Rel(cwd, path)
-			if err != nil {
-				t.Errorf("rel(%s,%s): %s", cwd, path, err)
-				return path
-			}
-			return rel
-		}
-
+		// path.cwd should be absolute and all others should be relative, as per documentation.
 		expected := map[string]string{
-			"cwd":          ".",
+			"cwd":          cwd,
 			"root":         "testdata/sourcebundle/planning/path_values/module",       // this is the root module of the component
 			"module":       "testdata/sourcebundle/planning/path_values/module",       // this is the root module
 			"child_root":   "testdata/sourcebundle/planning/path_values/module",       // should be the same for all modules
 			"child_module": "testdata/sourcebundle/planning/path_values/module/child", // this is the child module
 		}
 
 		actual := map[string]string{
-			"cwd":          normalizePath(component.PlannedOutputValues[addrs.OutputValue{Name: "cwd"}].AsString()),
-			"root":         normalizePath(component.PlannedOutputValues[addrs.OutputValue{Name: "root"}].AsString()),
-			"module":       normalizePath(component.PlannedOutputValues[addrs.OutputValue{Name: "module"}].AsString()),
-			"child_root":   normalizePath(component.PlannedOutputValues[addrs.OutputValue{Name: "child_root"}].AsString()),
-			"child_module": normalizePath(component.PlannedOutputValues[addrs.OutputValue{Name: "child_module"}].AsString()),
+			"cwd":          component.PlannedOutputValues[addrs.OutputValue{Name: "cwd"}].AsString(),
+			"root":         component.PlannedOutputValues[addrs.OutputValue{Name: "root"}].AsString(),
+			"module":       component.PlannedOutputValues[addrs.OutputValue{Name: "module"}].AsString(),
+			"child_root":   component.PlannedOutputValues[addrs.OutputValue{Name: "child_root"}].AsString(),
+			"child_module": component.PlannedOutputValues[addrs.OutputValue{Name: "child_module"}].AsString(),
 		}
 
 		if cmp.Diff(expected, actual) != "" {