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Implement context-based cancellation in `/lib/utils/prompt`, for MFA prompts. This fixes the following scenario: ```sh User has both OTP and U2F devices registered. $ tsh mfa ls Name Type Added at Last used ----- ---- ----------------------------- ----------------------------- otp TOTP Wed, 21 Apr 2021 19:41:44 UTC Wed, 21 Apr 2021 19:44:32 UTC usb-a U2F Wed, 21 Apr 2021 19:44:34 UTC Wed, 21 Apr 2021 19:44:34 UTC Add a new OTP device, using existing U2F device: $ tsh mfa add Choose device type [TOTP, U2F]: totp Enter device name: otp2 Tap any *registered* security key or enter a code from a *registered* OTP device: <tap> # <- First OTP prompt here Open your TOTP app and create a new manual entry with these fields: Name: awly@localhost:3080 Issuer: Teleport Algorithm: SHA1 Number of digits: 6 Period: 30s Secret: 3UD42X2NN7EEZ6LUPG6NFMNOLDY6AJTS Once created, enter an OTP code generated by the app: 607738 # <- Second OTP prompt here MFA device "otp2" added. ``` Before this PR, the first OTP prompt (for `*registered* device`) would hang in the background. The OTP code from the newly-registered device is prompted later, but any text written ends up going to the first prompt. After this PR, the first prompt is canceled and the code from a new device goes to the second prompt as intended. Note: this is implemented using pure Go code (background goroutine consuming `os.Stdin`) rather than syscalls (e.g. `poll` or `select`) for portability.
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Andrew Lytvynov
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May 3, 2021
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,143 @@ | ||
| /* | ||
| Copyright 2021 Gravitational, Inc. | ||
| Licensed under the Apache License, Version 2.0 (the "License"); | ||
| you may not use this file except in compliance with the License. | ||
| You may obtain a copy of the License at | ||
| http://www.apache.org/licenses/LICENSE-2.0 | ||
| Unless required by applicable law or agreed to in writing, software | ||
| distributed under the License is distributed on an "AS IS" BASIS, | ||
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| See the License for the specific language governing permissions and | ||
| limitations under the License. | ||
| */ | ||
|
|
||
| package prompt | ||
|
|
||
| import ( | ||
| "context" | ||
| "errors" | ||
| "io" | ||
| "os" | ||
| "sync" | ||
| ) | ||
|
|
||
| var ( | ||
| stdinOnce = &sync.Once{} | ||
| stdin *ContextReader | ||
| ) | ||
|
|
||
| // Stdin returns a singleton ContextReader wrapped around os.Stdin. | ||
| // | ||
| // os.Stdin should not be used directly after the first call to this function | ||
| // to avoid losing data. Closing this ContextReader will prevent all future | ||
| // reads for all callers. | ||
| func Stdin() *ContextReader { | ||
| stdinOnce.Do(func() { | ||
| stdin = NewContextReader(os.Stdin) | ||
| }) | ||
| return stdin | ||
| } | ||
|
|
||
| // ErrReaderClosed is returned from ContextReader.Read after it was closed. | ||
| var ErrReaderClosed = errors.New("ContextReader has been closed") | ||
|
|
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| // ContextReader is a wrapper around io.Reader where each individual | ||
| // ReadContext call can be canceled using a context. | ||
| type ContextReader struct { | ||
| r io.Reader | ||
| data chan []byte | ||
| close chan struct{} | ||
|
|
||
| mu sync.RWMutex | ||
| err error | ||
| } | ||
|
|
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| // NewContextReader creates a new ContextReader wrapping r. Callers should not | ||
| // use r after creating this ContextReader to avoid loss of data (the last read | ||
| // will be lost). | ||
| // | ||
| // Callers are responsible for closing the ContextReader to release associated | ||
| // resources. | ||
| func NewContextReader(r io.Reader) *ContextReader { | ||
| cr := &ContextReader{ | ||
| r: r, | ||
| data: make(chan []byte), | ||
| close: make(chan struct{}), | ||
| } | ||
| go cr.read() | ||
| return cr | ||
| } | ||
|
|
||
| func (r *ContextReader) setErr(err error) { | ||
| r.mu.Lock() | ||
| defer r.mu.Unlock() | ||
| if r.err != nil { | ||
| // Keep only the first encountered error. | ||
| return | ||
| } | ||
| r.err = err | ||
| } | ||
|
|
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| func (r *ContextReader) getErr() error { | ||
| r.mu.RLock() | ||
| defer r.mu.RUnlock() | ||
| return r.err | ||
| } | ||
|
|
||
| func (r *ContextReader) read() { | ||
| defer close(r.data) | ||
|
|
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| for { | ||
| // Allocate a new buffer for every read because we need to send it to | ||
| // another goroutine. | ||
| buf := make([]byte, 4*1024) // 4kB, matches Linux page size. | ||
| n, err := r.r.Read(buf) | ||
| r.setErr(err) | ||
| buf = buf[:n] | ||
| if n == 0 { | ||
| return | ||
| } | ||
| select { | ||
| case <-r.close: | ||
| return | ||
| case r.data <- buf: | ||
| } | ||
| } | ||
| } | ||
|
|
||
| // ReadContext returns the next chunk of output from the reader. If ctx is | ||
| // canceled before any data is available, ReadContext will return too. If r | ||
| // was closed, ReadContext will return immediately with ErrReaderClosed. | ||
| func (r *ContextReader) ReadContext(ctx context.Context) ([]byte, error) { | ||
| select { | ||
| case <-ctx.Done(): | ||
| return nil, ctx.Err() | ||
| case <-r.close: | ||
| // Close was called, unblock immediately. | ||
| // r.data might still be blocked if it's blocked on the Read call. | ||
| return nil, r.getErr() | ||
| case buf, ok := <-r.data: | ||
| if !ok { | ||
| // r.data was closed, so the read goroutine has finished. | ||
| // No more data will be available, return the latest error. | ||
| return nil, r.getErr() | ||
| } | ||
| return buf, nil | ||
| } | ||
| } | ||
|
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||
| // Close releases the background resources of r. All ReadContext calls will | ||
| // unblock immediately. | ||
| func (r *ContextReader) Close() { | ||
| select { | ||
| case <-r.close: | ||
| // Already closed, do nothing. | ||
| return | ||
| default: | ||
| close(r.close) | ||
| r.setErr(ErrReaderClosed) | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,76 @@ | ||
| /* | ||
| Copyright 2021 Gravitational, Inc. | ||
| Licensed under the Apache License, Version 2.0 (the "License"); | ||
| you may not use this file except in compliance with the License. | ||
| You may obtain a copy of the License at | ||
| http://www.apache.org/licenses/LICENSE-2.0 | ||
| Unless required by applicable law or agreed to in writing, software | ||
| distributed under the License is distributed on an "AS IS" BASIS, | ||
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| See the License for the specific language governing permissions and | ||
| limitations under the License. | ||
| */ | ||
|
|
||
| package prompt | ||
|
|
||
| import ( | ||
| "context" | ||
| "io" | ||
| "testing" | ||
|
|
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| "github.com/stretchr/testify/require" | ||
| ) | ||
|
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| func TestContextReader(t *testing.T) { | ||
| pr, pw := io.Pipe() | ||
| t.Cleanup(func() { pr.Close() }) | ||
| t.Cleanup(func() { pw.Close() }) | ||
|
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| write := func(t *testing.T, s string) { | ||
| _, err := pw.Write([]byte(s)) | ||
| require.NoError(t, err) | ||
| } | ||
| ctx := context.Background() | ||
|
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| r := NewContextReader(pr) | ||
|
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| t.Run("simple read", func(t *testing.T) { | ||
| go write(t, "hello") | ||
| buf, err := r.ReadContext(ctx) | ||
| require.NoError(t, err) | ||
| require.Equal(t, string(buf), "hello") | ||
| }) | ||
|
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| t.Run("cancelled read", func(t *testing.T) { | ||
| cancelCtx, cancel := context.WithCancel(ctx) | ||
| go cancel() | ||
| buf, err := r.ReadContext(cancelCtx) | ||
| require.ErrorIs(t, err, context.Canceled) | ||
| require.Empty(t, buf) | ||
|
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| go write(t, "after cancel") | ||
| buf, err = r.ReadContext(ctx) | ||
| require.NoError(t, err) | ||
| require.Equal(t, string(buf), "after cancel") | ||
| }) | ||
|
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| t.Run("close underlying reader", func(t *testing.T) { | ||
| go func() { | ||
| write(t, "before close") | ||
| pw.CloseWithError(io.EOF) | ||
| }() | ||
|
|
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| // Read the last chunk of data successfully. | ||
| buf, err := r.ReadContext(ctx) | ||
| require.NoError(t, err) | ||
| require.Equal(t, string(buf), "before close") | ||
|
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| // Next read fails because underlying reader is closed. | ||
| buf, err = r.ReadContext(ctx) | ||
| require.ErrorIs(t, err, io.EOF) | ||
| require.Empty(t, buf) | ||
| }) | ||
| } |
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