control/controlhttp, util/multierr: deduplicate context errorsandrew/context-dedup-errors

Add a new function to the util/multierr package that allows
deduplicating the `context.Canceled` and `context.DeadlineExceeded`
errors from a slice, and then use that to reduce log spam in
controlhttp.

Updates #14233

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I5a121124fe27c4449705ba35de6faf83665db5fe

3 files changed, 94 insertions, 1 deletions

diff --git a/control/controlhttp/client.go b/control/controlhttp/client.go
index 9b1d5a1a5..bf8d973e7 100644
--- a/control/controlhttp/client.go
+++ b/control/controlhttp/client.go
@@ -246,7 +246,7 @@ func (a *Dialer) dial(ctx context.Context) (*ClientConn, error) {
 		results[i].conn = nil // so we don't close it in the defer
 		return conn, nil
 	}
-	merr := multierr.New(errs...)
+	merr := multierr.New(multierr.DeduplicateContextErrors(errs)...)
 
 	// If we get here, then we didn't get anywhere with our dial plan; fall back to just using DNS.
 	a.logf("controlhttp: failed dialing using DialPlan, falling back to DNS; errs=%s", merr.Error())
diff --git a/util/multierr/multierr.go b/util/multierr/multierr.go
index 93ca068f5..42be001c6 100644
--- a/util/multierr/multierr.go
+++ b/util/multierr/multierr.go
@@ -6,6 +6,7 @@
 package multierr
 
 import (
+	"context"
 	"errors"
 	"slices"
 	"strings"
@@ -134,3 +135,40 @@ func Range(err error, fn func(error) bool) bool {
 	}
 	return true
 }
+
+// DeduplicateContextErrors returns a new slice of errors with at most one
+// occurrence of each [context.Canceled] or [context.DeadlineExceeded], if one
+// or more of them are present in the input slice.
+//
+// All other non-nil errors are returned as-is; nil errors are skipped.
+func DeduplicateContextErrors(errs []error) []error {
+	// preserve nil/non-nil distinction
+	if errs == nil {
+		return nil
+	} else if len(errs) == 0 {
+		return []error{}
+	}
+
+	var (
+		ret                      []error
+		sawCanceled, sawDeadline bool
+	)
+	for _, err := range errs {
+		if err == nil {
+			continue
+		}
+		if errors.Is(err, context.Canceled) {
+			if sawCanceled {
+				continue
+			}
+			sawCanceled = true
+		} else if errors.Is(err, context.DeadlineExceeded) {
+			if sawDeadline {
+				continue
+			}
+			sawDeadline = true
+		}
+		ret = append(ret, err)
+	}
+	return ret
+}
diff --git a/util/multierr/multierr_test.go b/util/multierr/multierr_test.go
index de7721a66..ea8c56637 100644
--- a/util/multierr/multierr_test.go
+++ b/util/multierr/multierr_test.go
@@ -4,6 +4,7 @@
 package multierr_test
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"io"
@@ -107,6 +108,60 @@ func TestRange(t *testing.T) {
 	})), want)
 }
 
+func TestDeduplicateContextErrors(t *testing.T) {
+	testError := errors.New("test error")
+
+	tests := []struct {
+		name  string
+		input []error
+		want  []error
+	}{
+		{name: "nil", input: nil, want: nil},
+		{name: "empty", input: []error{}, want: []error{}},
+		{name: "single", input: []error{testError}, want: []error{testError}},
+		{
+			name:  "duplicate_non_context",
+			input: []error{testError, testError},
+			want:  []error{testError, testError},
+		},
+		{
+			name:  "single_context",
+			input: []error{context.Canceled},
+			want:  []error{context.Canceled},
+		},
+		{
+			name:  "duplicate_context",
+			input: []error{testError, context.Canceled, context.Canceled},
+			want:  []error{testError, context.Canceled},
+		},
+		{
+			name: "duplicate_context_mixed",
+			input: []error{
+				testError,
+				context.Canceled,
+				context.Canceled,
+				testError,
+				context.DeadlineExceeded,
+				context.DeadlineExceeded,
+			},
+			want: []error{
+				testError,
+				context.Canceled,
+				testError,
+				context.DeadlineExceeded,
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := multierr.DeduplicateContextErrors(tt.input)
+			if diff := cmp.Diff(tt.want, got, cmpopts.EquateErrors()); diff != "" {
+				t.Errorf("DeduplicateContextErrors() mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
 var sink error
 
 func BenchmarkEmpty(b *testing.B) {