cmd/go: check case-insensitive path collisions for go mod vendor.

Fixes: #38571
Change-Id: Iec1cd1532ff17f7d943149f9b6a79e7fd419d179
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/570775
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: samthanawalla

src/cmd/go/internal/load/pkg.go

@@ -1937,6 +1937,8 @@ func (p *Package) load(ctx context.Context, opts PackageOpts, path string, stk *
 	}
 
 	// Check for case-insensitive collisions of import paths.
+	// If modifying, consider changing checkPathCollisions() in
+	// src/cmd/go/internal/modcmd/vendor.go
 	fold := str.ToFold(p.ImportPath)
 	if other := foldPath[fold]; other == "" {
 		foldPath[fold] = p.ImportPath

src/cmd/go/internal/modcmd/vendor.go

@@ -111,6 +111,7 @@ func RunVendor(ctx context.Context, vendorE bool, vendorO string, args []string)
 		}
 		modpkgs[m] = append(modpkgs[m], pkg)
 	}
+	checkPathCollisions(modpkgs)
 
 	includeAllReplacements := false
 	includeGoVersions := false
@@ -492,3 +493,19 @@ func copyDir(dst, src string, match func(dir string, info fs.DirEntry) bool, cop
 		}
 	}
 }
+
+// checkPathCollisions will fail if case-insensitive collisions are present.
+// The reason why we do this check in go mod vendor is to keep consistentcy
+// with go build. If modifying, consider changing load() in
+// src/cmd/go/internal/load/pkg.go
+func checkPathCollisions(modpkgs map[module.Version][]string) {
+	var foldPath = make(map[string]string, len(modpkgs))
+	for m := range modpkgs {
+		fold := str.ToFold(m.Path)
+		if other := foldPath[fold]; other == "" {
+			foldPath[fold] = m.Path
+		} else if other != m.Path {
+			base.Fatalf("go.mod: case-insensitive import collision: %q and %q", m.Path, other)
+		}
+	}
+}

src/cmd/go/testdata/script/mod_vendor_collision.txt

@@ -0,0 +1,38 @@
+! go build
+stderr 'case-insensitive import collision'
+
+! go mod vendor
+stderr 'case-insensitive import collision'
+
+-- foo.go --
+package main
+
+import (
+	_ "example.com/Foo"
+	_ "example.com/foo"
+)
+
+func main() {}
+-- go.mod --
+module play.ground
+
+go 1.14
+
+require (
+	example.com/foo v0.1.0
+	example.com/Foo v0.1.0
+)
+
+replace (
+	example.com/foo => ./foo
+	example.com/Foo => ./foo_alt
+)
+-- foo/go.mod --
+module example.com/foo
+-- foo/foo.go --
+package foo
+
+-- foo_alt/go.mod --
+module example.com/Foo
+-- foo_alt/foo.go --
+package Foo