cmd/go, x/tools/gopls: misleading errors for invalid import

[findleyr]

Just noticed this while refactoring import paths:

In this case, I'm in the process of moving the 'tag' package, and the old location no longer exists. I believe the error message is due to the logic here: https://cs.opensource.google/go/x/tools/+/master:internal/gcimporter/iimport.go;l=218;drc=fa556487c5c2be818dd2bab43e16f1afa06f8f89

The problem is that we're now importing packages with broken metadata, and invalid imports appear to have package path "".

[findleyr]

Oh, this appears to simply be a go/packages bug, as I can reproduce without gopls, using x/tools/go/packages/gopackages.

Note that after removing the tag package, the "CompiledGoFiles" field is simply wrong. It picks up the first compiled go file of the jsonrpc2 package.

CC @adonovan @matloob @bcmills

I have not yet reproduced this with go list alone, so it may be a bug in the go list driver)

> gopackages -json golang.org/x/tools/internal/jsonrpc2 golang.org/x/tools/internal/event/tag
{
	"ID": "golang.org/x/tools/internal/event/tag",
	"PkgPath": "golang.org/x/tools/internal/event/tag",
	"Errors": [
		{
			"Pos": "../internal/jsonrpc2/conn.go:16:2",
			"Msg": "no required module provides package golang.org/x/tools/internal/event/tag; to add it:\n\tgo get golang.org/x/tools/internal/event/tag",
			"Kind": 1
		}
	],
	"GoFiles": [
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/conn.go"
	],
	"CompiledGoFiles": [
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/conn.go"
	]
}{
	"ID": "golang.org/x/tools/internal/jsonrpc2",
	"Name": "jsonrpc2",
	"PkgPath": "golang.org/x/tools/internal/jsonrpc2",
	"GoFiles": [
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/conn.go",
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/handler.go",
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/jsonrpc2.go",
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/messages.go",
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/serve.go",
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/stream.go",
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/wire.go"
	],
	"CompiledGoFiles": [
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/conn.go",
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/handler.go",
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/jsonrpc2.go",
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/messages.go",
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/serve.go",
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/stream.go",
		"/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2/wire.go"
	],
	"Imports": {
		"bufio": "bufio",
		"context": "context",
		"encoding/json": "encoding/json",
		"errors": "errors",
		"fmt": "fmt",
		"golang.org/x/tools/internal/event": "golang.org/x/tools/internal/event",
		"golang.org/x/tools/internal/event/label": "golang.org/x/tools/internal/event/label",
		"golang.org/x/tools/internal/event/tag": "golang.org/x/tools/internal/event/tag",
		"io": "io",
		"math": "math",
		"net": "net",
		"os": "os",
		"strconv": "strconv",
		"strings": "strings",
		"sync": "sync",
		"sync/atomic": "sync/atomic",
		"time": "time"
	}
}

[findleyr]

Actually, this is a Go command bug. Compare the two go list invocations below. When the jsonrpc2 package is included, the error for internal/event/tag erroneously uses a position from the jsonrpc2 package.

go/packages interprets this error to synthesize missing GoFiles, due to #39986.

> go list -e -json -compiled=true -test=false -export=false -find=false -- golang.org/x/tools/internal/event/tag
{
	"ImportPath": "golang.org/x/tools/internal/event/tag",
	"Match": [
		"golang.org/x/tools/internal/event/tag"
	],
	"Incomplete": true,
	"Error": {
		"ImportStack": [],
		"Pos": "",
		"Err": "no required module provides package golang.org/x/tools/internal/event/tag; to add it:\n\tgo get golang.org/x/tools/internal/event/tag"
	}
}

vs.

> go list -e -json -compiled=true -test=false -export=false -find=false -- golang.org/x/tools/internal/jsonrpc2 golang.org/x/tools/internal/event/tag
{
	"Dir": "/usr/local/google/home/rfindley/src/tools2/internal/jsonrpc2",
	"ImportPath": "golang.org/x/tools/internal/jsonrpc2",
	"Name": "jsonrpc2",
	"Doc": "Package jsonrpc2 is a minimal implementation of the JSON RPC 2 spec.",
	"Root": "/usr/local/google/home/rfindley/src/tools2",
	"Module": {
		"Path": "golang.org/x/tools",
		"Main": true,
		"Dir": "/usr/local/google/home/rfindley/src/tools2",
		"GoMod": "/usr/local/google/home/rfindley/src/tools2/go.mod",
		"GoVersion": "1.18"
	},
	"Match": [
		"golang.org/x/tools/internal/jsonrpc2"
	],
	"Incomplete": true,
	"Stale": true,
	"StaleReason": "stale dependency: golang.org/x/tools/internal/event/tag",
	"GoFiles": [
		"conn.go",
		"handler.go",
		"jsonrpc2.go",
		"messages.go",
		"serve.go",
		"stream.go",
		"wire.go"
	],
	"CompiledGoFiles": [
		"conn.go",
		"handler.go",
		"jsonrpc2.go",
		"messages.go",
		"serve.go",
		"stream.go",
		"wire.go"
	],
	"Imports": [
		"bufio",
		"context",
		"encoding/json",
		"errors",
		"fmt",
		"golang.org/x/tools/internal/event",
		"golang.org/x/tools/internal/event/label",
		"golang.org/x/tools/internal/event/tag",
		"io",
		"math",
		"net",
		"os",
		"strconv",
		"strings",
		"sync",
		"sync/atomic",
		"time"
	],
	"Deps": [
		"bufio",
		"bytes",
		"context",
		"encoding",
		"encoding/base64",
		"encoding/binary",
		"encoding/json",
		"errors",
		"fmt",
		"golang.org/x/tools/internal/event",
		"golang.org/x/tools/internal/event/core",
		"golang.org/x/tools/internal/event/keys",
		"golang.org/x/tools/internal/event/label",
		"golang.org/x/tools/internal/event/tag",
		"internal/abi",
		"internal/bytealg",
		"internal/coverage/rtcov",
		"internal/cpu",
		"internal/fmtsort",
		"internal/goarch",
		"internal/godebug",
		"internal/goexperiment",
		"internal/goos",
		"internal/intern",
		"internal/itoa",
		"internal/nettrace",
		"internal/oserror",
		"internal/poll",
		"internal/race",
		"internal/reflectlite",
		"internal/safefilepath",
		"internal/singleflight",
		"internal/syscall/execenv",
		"internal/syscall/unix",
		"internal/testlog",
		"internal/unsafeheader",
		"io",
		"io/fs",
		"math",
		"math/bits",
		"net",
		"net/netip",
		"os",
		"path",
		"reflect",
		"runtime",
		"runtime/cgo",
		"runtime/internal/atomic",
		"runtime/internal/math",
		"runtime/internal/sys",
		"runtime/internal/syscall",
		"sort",
		"strconv",
		"strings",
		"sync",
		"sync/atomic",
		"syscall",
		"time",
		"unicode",
		"unicode/utf16",
		"unicode/utf8",
		"unsafe",
		"vendor/golang.org/x/net/dns/dnsmessage"
	],
	"DepsErrors": [
		{
			"ImportStack": [
				"golang.org/x/tools/internal/event/tag"
			],
			"Pos": "internal/jsonrpc2/conn.go:16:2",
			"Err": "no required module provides package golang.org/x/tools/internal/event/tag; to add it:\n\tgo get golang.org/x/tools/internal/event/tag"
		}
	],
	"TestGoFiles": [
		"serve_test.go"
	],
	"TestImports": [
		"context",
		"golang.org/x/tools/internal/stack/stacktest",
		"golang.org/x/tools/internal/testenv",
		"net",
		"sync",
		"testing",
		"time"
	],
	"XTestGoFiles": [
		"jsonrpc2_test.go",
		"wire_test.go"
	],
	"XTestImports": [
		"bytes",
		"context",
		"encoding/json",
		"flag",
		"fmt",
		"golang.org/x/tools/internal/event/export/eventtest",
		"golang.org/x/tools/internal/jsonrpc2",
		"golang.org/x/tools/internal/stack/stacktest",
		"net",
		"path",
		"reflect",
		"testing"
	]
}
{
	"ImportPath": "golang.org/x/tools/internal/event/tag",
	"Match": [
		"golang.org/x/tools/internal/event/tag"
	],
	"Incomplete": true,
	"Stale": true,
	"StaleReason": "build ID mismatch",
	"Error": {
		"ImportStack": [
			"golang.org/x/tools/internal/event/tag"
		],
		"Pos": "internal/jsonrpc2/conn.go:16:2",
		"Err": "no required module provides package golang.org/x/tools/internal/event/tag; to add it:\n\tgo get golang.org/x/tools/internal/event/tag"
	}
}