Replace os.IsNotExist and friends with errors.Is (#4969)

## Summary

- Replace all uses of `os.IsNotExist`, `os.IsExist`, and
`os.IsPermission` with their `errors.Is` equivalents (`errors.Is(err,
fs.ErrNotExist)`, etc.)
- Add `forbidigo` linter rules to reject future use of these functions
- The [Go documentation](https://pkg.go.dev/os#IsNotExist) recommends
`errors.Is` because it unwraps the error chain, while the `os.Is*`
functions only recognize errors returned by the `os` package directly

## Test plan

- [x] `go build ./...` passes
- [x] `make lint` passes with 0 issues

This pull request was AI-assisted by Isaac.

Author: pietern

.golangci.yaml

@@ -60,6 +60,12 @@ linters:
           msg: Use slices.Sort from the standard library instead.
         - pattern: 'sort\.Float64s'
           msg: Use slices.Sort from the standard library instead.
+        - pattern: 'os\.IsNotExist'
+          msg: Use errors.Is(err, fs.ErrNotExist) instead.
+        - pattern: 'os\.IsExist'
+          msg: Use errors.Is(err, fs.ErrExist) instead.
+        - pattern: 'os\.IsPermission'
+          msg: Use errors.Is(err, fs.ErrPermission) instead.
       analyze-types: true
     copyloopvar:
       check-alias: true

acceptance/acceptance_test.go

@@ -10,6 +10,7 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"io/fs"
 	"maps"
 	"net/http"
 	"os"
@@ -1486,7 +1487,7 @@ func setupTerraform(t *testing.T, cwd, buildDir string, repls *testdiff.Replacem
 
 func loadUserReplacements(t *testing.T, repls *testdiff.ReplacementsContext, tmpDir string) {
 	b, err := os.ReadFile(filepath.Join(tmpDir, userReplacementsFilename))
-	if os.IsNotExist(err) {
+	if errors.Is(err, fs.ErrNotExist) {
 		return
 	}
 	require.NoError(t, err)

acceptance/internal/config.go

@@ -1,7 +1,9 @@
 package internal
 
 import (
+	"errors"
 	"hash/fnv"
+	"io/fs"
 	"maps"
 	"os"
 	"path/filepath"
@@ -181,7 +183,7 @@ func FindConfigs(t *testing.T, dir string) []string {
 
 		dir = filepath.Dir(dir)
 
-		if err == nil || os.IsNotExist(err) {
+		if err == nil || errors.Is(err, fs.ErrNotExist) {
 			continue
 		}
 

bundle/direct/dstate/state.go

@@ -3,7 +3,9 @@ package dstate
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"strings"
@@ -118,7 +120,7 @@ func (db *DeploymentState) Open(path string) error {
 
 	data, err := os.ReadFile(path)
 	if err != nil {
-		if os.IsNotExist(err) {
+		if errors.Is(err, fs.ErrNotExist) {
 			// Create new database with serial=0, will be incremented to 1 in Finalize()
 			db.Data = NewDatabase("", 0)
 			db.Path = path

bundle/docsgen/main.go

@@ -1,7 +1,9 @@
 package main
 
 import (
+	"errors"
 	"fmt"
+	"io/fs"
 	"log"
 	"os"
 	"path"
@@ -30,7 +32,7 @@ func main() {
 	outputDir := path.Join(docsDir, "output")
 	templatesDir := path.Join(docsDir, "templates")
 
-	if _, err := os.Stat(outputDir); os.IsNotExist(err) {
+	if _, err := os.Stat(outputDir); errors.Is(err, fs.ErrNotExist) {
 		if err := os.MkdirAll(outputDir, 0o755); err != nil {
 			log.Fatal(err)
 		}

bundle/statemgmt/state_pull.go

@@ -59,7 +59,7 @@ func (s *StateDesc) HasRemoteTerraformState() bool {
 func localRead(ctx context.Context, fullPath string, engine engine.EngineType) *StateDesc {
 	content, err := os.ReadFile(fullPath)
 	if err != nil {
-		if !os.IsNotExist(err) {
+		if !errors.Is(err, fs.ErrNotExist) {
 			logdiag.LogError(ctx, fmt.Errorf("reading %s: %w", filepath.ToSlash(fullPath), err))
 		}
 		return nil

cmd/apps/dev.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io/fs"
 	"net"
 	"net/url"
 	"os"
@@ -144,7 +145,7 @@ Examples:
 			ctx := cmd.Context()
 
 			// Validate client path early (before any network calls)
-			if _, err := os.Stat(clientPath); os.IsNotExist(err) {
+			if _, err := os.Stat(clientPath); errors.Is(err, fs.ErrNotExist) {
 				return fmt.Errorf("client directory not found: %s", clientPath)
 			}
 

cmd/apps/import.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"slices"
@@ -171,7 +172,7 @@ Examples:
 			// Check if output directory already exists
 			if _, err := os.Stat(outputDir); err == nil {
 				return fmt.Errorf("directory '%s' already exists. Please remove it or choose a different output directory", outputDir)
-			} else if !os.IsNotExist(err) {
+			} else if !errors.Is(err, fs.ErrNotExist) {
 				return fmt.Errorf("failed to check if directory exists: %w", err)
 			}
 

cmd/apps/init.go

@@ -791,10 +791,10 @@ func runCreate(ctx context.Context, opts createOptions) error {
 
 	// Check for generic subdirectory first (default for multi-template repos)
 	templateDir := filepath.Join(resolvedPath, "generic")
-	if _, err := os.Stat(templateDir); os.IsNotExist(err) {
+	if _, err := os.Stat(templateDir); errors.Is(err, fs.ErrNotExist) {
 		// Fall back to the provided path directly
 		templateDir = resolvedPath
-		if _, err := os.Stat(templateDir); os.IsNotExist(err) {
+		if _, err := os.Stat(templateDir); errors.Is(err, fs.ErrNotExist) {
 			return fmt.Errorf("template not found at %s (also checked %s/generic)", resolvedPath, resolvedPath)
 		}
 	}

cmd/apps/manifest.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 
@@ -46,9 +47,9 @@ func runManifestOnly(ctx context.Context, templatePath, branch, version string)
 	}
 
 	templateDir := filepath.Join(resolvedPath, "generic")
-	if _, err := os.Stat(templateDir); os.IsNotExist(err) {
+	if _, err := os.Stat(templateDir); errors.Is(err, fs.ErrNotExist) {
 		templateDir = resolvedPath
-		if _, err := os.Stat(templateDir); os.IsNotExist(err) {
+		if _, err := os.Stat(templateDir); errors.Is(err, fs.ErrNotExist) {
 			return fmt.Errorf("template not found at %s (also checked %s/generic)", resolvedPath, resolvedPath)
 		}
 	}