fix combination of --fix and --path-prefix

This combination used to fail because the fixer was executed after the path
prefixer. Then the file paths that it processed didn't match the current
working directory, leading to:

     ERRO Failed to fix issues in file test/fix_sample/main.go: failed to get file bytes for test/fix_sample/main.go: can't read file test/fix_sample/main.go: open test/fix_sample/main.go: no such file or directory

Making the fixer a normal processor and moving it before the path prefixer
avoids this problem.

Author: pohly

Diff for: pkg/commands/run.go

@@ -23,7 +23,6 @@ import (
 	"github.com/golangci/golangci-lint/pkg/packages"
 	"github.com/golangci/golangci-lint/pkg/printers"
 	"github.com/golangci/golangci-lint/pkg/result"
-	"github.com/golangci/golangci-lint/pkg/result/processors"
 )
 
 const defaultFileMode = 0644
@@ -365,13 +364,7 @@ func (e *Executor) runAnalysis(ctx context.Context, args []string) ([]result.Iss
 		return nil, err
 	}
 
-	issues, err := runner.Run(ctx, lintersToRun, lintCtx)
-	if err != nil {
-		return nil, err
-	}
-
-	fixer := processors.NewFixer(e.cfg, e.log, e.fileCache)
-	return fixer.Process(issues), nil
+	return runner.Run(ctx, lintersToRun, lintCtx)
 }
 
 func (e *Executor) setOutputToDevNull() (savedStdout, savedStderr *os.File) {

Diff for: pkg/fsutils/linecache.go

@@ -19,6 +19,8 @@ func NewLineCache(fc *FileCache) *LineCache {
 	}
 }
 
+func (lc *LineCache) GetFileCache() *FileCache { return lc.fileCache }
+
 // GetLine returns the index1-th (1-based index) line from the file on filePath
 func (lc *LineCache) GetLine(filePath string, index1 int) (string, error) {
 	if index1 == 0 { // some linters, e.g. gosec can do it: it really means first line

Diff for: pkg/lint/runner.go

@@ -97,7 +97,9 @@ func NewRunner(cfg *config.Config, log logutils.Log, goenv *goutil.Env, es *lint
 			processors.NewMaxFromLinter(cfg.Issues.MaxIssuesPerLinter, log.Child(logutils.DebugKeyMaxFromLinter), cfg),
 			processors.NewSourceCode(lineCache, log.Child(logutils.DebugKeySourceCode)),
 			processors.NewPathShortener(),
-			getSeverityRulesProcessor(&cfg.Severity, log, files),
+			// The fixer still needs to see paths for the issues that are relative to the current directory.
+			processors.NewFixer(cfg, log, lineCache.GetFileCache()),
+			// Now we can modify the issues for output.
 			processors.NewPathPrefixer(cfg.Output.PathPrefix),
 			processors.NewSortResults(cfg),
 		},

Diff for: pkg/result/processors/fixer.go

@@ -36,9 +36,9 @@ func (f Fixer) printStat() {
 	f.sw.PrintStages()
 }
 
-func (f Fixer) Process(issues []result.Issue) []result.Issue {
+func (f Fixer) Process(issues []result.Issue) ([]result.Issue, error) {
 	if !f.cfg.Issues.NeedFix {
-		return issues
+		return issues, nil
 	}
 
 	outIssues := make([]result.Issue, 0, len(issues))
@@ -67,9 +67,17 @@ func (f Fixer) Process(issues []result.Issue) []result.Issue {
 	}
 
 	f.printStat()
-	return outIssues
+	return outIssues, nil
 }
 
+func (f Fixer) Name() string {
+	return "fixer"
+}
+
+func (f Fixer) Finish() {}
+
+var _ Processor = Fixer{}
+
 func (f Fixer) fixIssuesInFile(filePath string, issues []result.Issue) error {
 	// TODO: don't read the whole file into memory: read line by line;
 	// can't just use bufio.scanner: it has a line length limit