collectors: detect DeadlineExceeded from gRPC status code

IsDeadlineExceeded missed a deadline error whose gRPC code is
DeadlineExceeded but whose description is "stream terminated by
RST_STREAM with error code: CANCEL" rather than "context deadline
exceeded". Neither existing check caught it: status.FromContextError
only matches errors that are/wrap context.DeadlineExceeded, and the
string fallback looked for the literal "context deadline exceeded"
description.

As a result the error slipped past the tolerance guard in the
collectors, got forwarded to errChan, and killed lndmon on a transient
scrape-cycle timeout.

Check the actual gRPC status code via status.FromError (which also
unwraps wrapped errors through errors.As), so any DeadlineExceeded
status is tolerated regardless of its description text. This is shared
by every collector that calls IsDeadlineExceeded.

Add errors_test.go covering the RST_STREAM variant (plain and wrapped),
the original context-deadline shapes, and non-deadline errors.

Add a "test" Makefile target that runs "go test -v ./..." so the unit
tests can be run with "make test", and it shows up in "make list".

Author: ZZiigguurraatt

Makefile

@@ -9,6 +9,7 @@ GO_BIN := ${GOPATH}/bin
 LINT_BIN := $(GO_BIN)/golangci-lint
 
 GOBUILD := go build -v
+GOTEST := go test -v
 
 GOFILES_NOVENDOR = $(shell find . -type f -name '*.go' -not -path "./vendor/*")
 GOLIST := go list -deps $(PKG)/... | grep '$(PKG)'| grep -v '/vendor/'
@@ -47,6 +48,14 @@ build:
 	@$(call print, "Building lndmon.")
 	$(GOBUILD) $(PKG)/cmd/lndmon
 
+# =======
+# TESTING
+# =======
+
+test:
+	@$(call print, "Running unit tests.")
+	$(GOTEST) ./...
+
 # =========
 # UTILITIES
 # =========

collectors/errors.go

@@ -26,6 +26,19 @@ func IsDeadlineExceeded(err error) bool {
 		return false
 	}
 
+	// If the error is (or wraps) a gRPC status error, check its code
+	// directly. This catches server-side deadlines that arrive with a
+	// non-standard description, e.g. "stream terminated by RST_STREAM with
+	// error code: CANCEL", where the code is DeadlineExceeded but the
+	// description doesn't match context.DeadlineExceeded.
+	if st, ok := status.FromError(err); ok &&
+		st.Code() == codes.DeadlineExceeded {
+
+		return true
+	}
+
+	// The error may also be a bare context error (client-side deadline)
+	// that isn't wrapped in a gRPC status.
 	st := status.FromContextError(err)
 	if st.Code() == codes.DeadlineExceeded {
 		return true

collectors/errors_test.go

@@ -0,0 +1,82 @@
+package collectors
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"testing"
+
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+)
+
+// TestIsDeadlineExceeded verifies that IsDeadlineExceeded recognizes the
+// various shapes a deadline error can take, most importantly a server-side
+// gRPC status error whose code is DeadlineExceeded but whose description does
+// not mention "context deadline exceeded".
+func TestIsDeadlineExceeded(t *testing.T) {
+	tests := []struct {
+		name string
+		err  error
+		want bool
+	}{
+		{
+			name: "nil error",
+			err:  nil,
+			want: false,
+		},
+		{
+			name: "bare context deadline",
+			err:  context.DeadlineExceeded,
+			want: true,
+		},
+		{
+			name: "grpc status with context deadline desc",
+			err: status.Error(
+				codes.DeadlineExceeded,
+				context.DeadlineExceeded.Error(),
+			),
+			want: true,
+		},
+		{
+			name: "grpc RST_STREAM deadline",
+			err: status.Error(
+				codes.DeadlineExceeded,
+				"stream terminated by RST_STREAM with error "+
+					"code: CANCEL",
+			),
+			want: true,
+		},
+		{
+			name: "wrapped grpc RST_STREAM deadline",
+			err: fmt.Errorf("WalletCollector WalletBalance failed "+
+				"with: %w", status.Error(
+				codes.DeadlineExceeded,
+				"stream terminated by RST_STREAM with error "+
+					"code: CANCEL",
+			)),
+			want: true,
+		},
+		{
+			name: "unrelated grpc error",
+			err: status.Error(
+				codes.Unavailable, "connection refused",
+			),
+			want: false,
+		},
+		{
+			name: "unrelated plain error",
+			err:  errors.New("something else failed"),
+			want: false,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := IsDeadlineExceeded(tc.err); got != tc.want {
+				t.Fatalf("IsDeadlineExceeded(%v) = %v, want %v",
+					tc.err, got, tc.want)
+			}
+		})
+	}
+}