smarter detection for the go.mod file location

gomodguard.go

@@ -1,11 +1,14 @@
 package gomodguard
 
 import (
+	"bytes"
 	"fmt"
 	"go/parser"
 	"go/token"
 	"io/ioutil"
 	"log"
+	"os"
+	"os/exec"
 	"strings"
 
 	"golang.org/x/mod/modfile"
@@ -14,7 +17,7 @@ import (
 var (
 	blockedReasonNotInAllowedList = "import of package `%s` is blocked because the module is not in the allowed modules list."
 	blockedReasonInBlockedList    = "import of package `%s` is blocked because the module is in the blocked modules list."
-	goModFile                     = "go.mod"
+	goModFilename                 = "go.mod"
 )
 
 // Recommendations are alternative modules to use and a reason why.
@@ -191,18 +194,16 @@ type Processor struct {
 
 // NewProcessor will create a Processor to lint blocked packages.
 func NewProcessor(config Configuration, logger *log.Logger) (*Processor, error) {
-	moddata, err := ioutil.ReadFile(goModFile)
+	goModFileBytes, err := loadGoModFile()
 	if err != nil {
-		errMsg := fmt.Sprintf("unable to read go.mod file: %s", err)
-		logger.Printf(errMsg)
+		errMsg := fmt.Sprintf("unable to read %s file: %s", goModFilename, err)
 
 		return nil, fmt.Errorf(errMsg)
 	}
 
-	mfile, err := modfile.Parse(goModFile, moddata, nil)
+	mfile, err := modfile.Parse(goModFilename, goModFileBytes, nil)
 	if err != nil {
-		errMsg := fmt.Sprintf("unable to parse go.mod file: %s", err)
-		logger.Printf(errMsg)
+		errMsg := fmt.Sprintf("unable to parse %s file: %s", goModFilename, err)
 
 		return nil, fmt.Errorf(errMsg)
 	}
@@ -231,7 +232,8 @@ func (p *Processor) ProcessFiles(filenames []string) []Result {
 		pluralModuleMsg = ""
 	}
 
-	p.logger.Printf("info: found `%d` blocked module%s in the go.mod file, %+v", len(p.blockedModulesFromModFile), pluralModuleMsg, p.blockedModulesFromModFile)
+	p.logger.Printf("info: found `%d` blocked module%s in the %s file, %+v",
+		len(p.blockedModulesFromModFile), pluralModuleMsg, goModFilename, p.blockedModulesFromModFile)
 
 	for _, filename := range filenames {
 		data, err := ioutil.ReadFile(filename)
@@ -344,3 +346,23 @@ func (p *Processor) isBlockedPackageFromModFile(pkg string) bool {
 
 	return false
 }
+
+func loadGoModFile() ([]byte, error) {
+	cmd := exec.Command("go", "list", "-m", "-f", "{{.GoMod}}")
+	stdout, _ := cmd.StdoutPipe()
+	_ = cmd.Start()
+
+	goModFileLocation := ""
+
+	if stdout != nil {
+		buf := new(bytes.Buffer)
+		_, _ = buf.ReadFrom(stdout)
+		goModFileLocation = strings.TrimSpace(buf.String())
+	}
+
+	if _, err := os.Stat(goModFileLocation); os.IsNotExist(err) {
+		return ioutil.ReadFile(goModFilename)
+	}
+
+	return ioutil.ReadFile(goModFileLocation)
+}