Make TryInsert functions within the packages module try to insert first

The TryInsert* functions within the packages models make incorrect assumptions
about transactional isolation within most databases. It is perfectly possible for
a SELECT to return nothing but an INSERT fail with a duplicate in most DBs as it
is only INSERT that the locking occurs.

This PR changes the code to simply try to insert first and if there is an error
then attempt to SELECT from the table. If the SELECT works then the INSERT error
is assumed to have been related to the unique constraint failure.

This technique avoids us having to parse the error returned from the DBs as
these are varied and different.

If the SELECT fails then the INSERT error is returned to the user.

Fix go-gitea#19586

Signed-off-by: Andrew Thornton <art27@cantab.net>

models/packages/package.go

@@ -128,14 +128,11 @@ func TryInsertPackage(ctx context.Context, p *Package) (*Package, error) {
 		LowerName: p.LowerName,
 	}
 
-	has, err := e.Get(key)
-	if err != nil {
-		return nil, err
-	}
-	if has {
-		return key, ErrDuplicatePackage
-	}
-	if _, err = e.Insert(p); err != nil {
+	if _, err := e.Insert(p); err != nil {
+		// Try to get the key again
+		if has, _ := e.Get(key); has {
+			return key, ErrDuplicatePackage
+		}
 		return nil, err
 	}
 	return p, nil

models/packages/package_file.go

@@ -53,14 +53,10 @@ func TryInsertFile(ctx context.Context, pf *PackageFile) (*PackageFile, error) {
 		CompositeKey: pf.CompositeKey,
 	}
 
-	has, err := e.Get(key)
-	if err != nil {
-		return nil, err
-	}
-	if has {
-		return pf, ErrDuplicatePackageFile
-	}
-	if _, err = e.Insert(pf); err != nil {
+	if _, err := e.Insert(pf); err != nil {
+		if has, _ := e.Get(key); has {
+			return pf, ErrDuplicatePackageFile
+		}
 		return nil, err
 	}
 	return pf, nil

models/packages/package_version.go

@@ -46,14 +46,10 @@ func GetOrInsertVersion(ctx context.Context, pv *PackageVersion) (*PackageVersio
 		LowerVersion: pv.LowerVersion,
 	}
 
-	has, err := e.Get(key)
-	if err != nil {
-		return nil, err
-	}
-	if has {
-		return key, ErrDuplicatePackageVersion
-	}
-	if _, err = e.Insert(pv); err != nil {
+	if _, err := e.Insert(pv); err != nil {
+		if has, _ := e.Get(key); has {
+			return key, ErrDuplicatePackageVersion
+		}
 		return nil, err
 	}
 	return pv, nil