1 // Copyright 2023, Google Inc.
2 // All rights reserved.
3 //
4 // Redistribution and use in source and binary forms, with or without
5 // modification, are permitted provided that the following conditions are
6 // met:
7 //
8 // * Redistributions of source code must retain the above copyright
9 // notice, this list of conditions and the following disclaimer.
10 // * Redistributions in binary form must reproduce the above
11 // copyright notice, this list of conditions and the following disclaimer
12 // in the documentation and/or other materials provided with the
13 // distribution.
14 // * Neither the name of Google Inc. nor the names of its
15 // contributors may be used to endorse or promote products derived from
16 // this software without specific prior written permission.
17 //
18 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
19 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
20 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
21 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
22 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
23 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
24 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
25 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
26 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
27 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
28 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
29 30 // Package callctx provides helpers for storing and retrieving values out of
31 // [context.Context]. These values are used by our client libraries in various
32 // ways across the stack.
33 package callctx
34 35 import (
36 "context"
37 "fmt"
38 )
39 40 const (
41 // XGoogFieldMaskHeader is the canonical header key for the [System Parameter]
42 // that specifies the response read mask. The value(s) for this header
43 // must adhere to format described in [fieldmaskpb].
44 //
45 // [System Parameter]: https://cloud.google.com/apis/docs/system-parameters
46 // [fieldmaskpb]: https://google.golang.org/protobuf/types/known/fieldmaskpb
47 XGoogFieldMaskHeader = "x-goog-fieldmask"
48 49 headerKey = contextKey("header")
50 )
51 52 // contextKey is a private type used to store/retrieve context values.
53 type contextKey string
54 55 // HeadersFromContext retrieves headers set from [SetHeaders]. These headers
56 // can then be cast to http.Header or metadata.MD to send along on requests.
57 func HeadersFromContext(ctx context.Context) map[string][]string {
58 m, ok := ctx.Value(headerKey).(map[string][]string)
59 if !ok {
60 return nil
61 }
62 return m
63 }
64 65 // SetHeaders stores key value pairs in the returned context that can later
66 // be retrieved by [HeadersFromContext]. Values stored in this manner will
67 // automatically be retrieved by client libraries and sent as outgoing headers
68 // on all requests. keyvals should have a corresponding value for every key
69 // provided. If there is an odd number of keyvals this method will panic.
70 func SetHeaders(ctx context.Context, keyvals ...string) context.Context {
71 if len(keyvals)%2 != 0 {
72 panic(fmt.Sprintf("callctx: an even number of key value pairs must be provided, got %d", len(keyvals)))
73 }
74 h, ok := ctx.Value(headerKey).(map[string][]string)
75 if !ok {
76 h = make(map[string][]string)
77 } else {
78 h = cloneHeaders(h)
79 }
80 81 for i := 0; i < len(keyvals); i = i + 2 {
82 h[keyvals[i]] = append(h[keyvals[i]], keyvals[i+1])
83 }
84 return context.WithValue(ctx, headerKey, h)
85 }
86 87 // cloneHeaders makes a new key-value map while reusing the value slices.
88 // As such, new values should be appended to the value slice, and modifying
89 // indexed values is not thread safe.
90 //
91 // TODO: Replace this with maps.Clone when Go 1.21 is the minimum version.
92 func cloneHeaders(h map[string][]string) map[string][]string {
93 c := make(map[string][]string, len(h))
94 for k, v := range h {
95 vc := make([]string, len(v))
96 copy(vc, v)
97 c[k] = vc
98 }
99 return c
100 }
101