commands: micro optimization for source date epoch

Avoids the call to `os.Getenv` when it is unnecessary because it would
be overwritten anyway.

Removes the comments about moving environment variable parsing to a
method for use by library consumers. That method exists in containerd
and this set of code doesn't actually perform any parsing since the
parsing of this time is done within buildkit and not on the client.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

vendor/github.com/containerd/containerd/v2/pkg/epoch/context.go

@@ -0,0 +1,41 @@
+/*
+   Copyright The containerd Authors.
+
+   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 epoch
+
+import (
+	"context"
+	"time"
+)
+
+type (
+	epochKey struct{}
+)
+
+// WithSourceDateEpoch associates the context with the epoch.
+func WithSourceDateEpoch(ctx context.Context, tm *time.Time) context.Context {
+	return context.WithValue(ctx, epochKey{}, tm)
+}
+
+// FromContext returns the epoch associated with the context.
+// FromContext does not fall back to read the SOURCE_DATE_EPOCH env var.
+func FromContext(ctx context.Context) *time.Time {
+	v := ctx.Value(epochKey{})
+	if v == nil {
+		return nil
+	}
+	return v.(*time.Time)
+}

vendor/github.com/containerd/containerd/v2/pkg/epoch/epoch.go

@@ -0,0 +1,67 @@
+/*
+   Copyright The containerd Authors.
+
+   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 epoch provides SOURCE_DATE_EPOCH utilities.
+package epoch
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"time"
+)
+
+// SourceDateEpochEnv is the SOURCE_DATE_EPOCH env var.
+// See https://reproducible-builds.org/docs/source-date-epoch/
+const SourceDateEpochEnv = "SOURCE_DATE_EPOCH"
+
+// SourceDateEpoch returns the SOURCE_DATE_EPOCH env var as *time.Time.
+// If the env var is not set, SourceDateEpoch returns nil without an error.
+func SourceDateEpoch() (*time.Time, error) {
+	v, ok := os.LookupEnv(SourceDateEpochEnv)
+	if !ok || v == "" {
+		return nil, nil // not an error
+	}
+	t, err := ParseSourceDateEpoch(v)
+	if err != nil {
+		return nil, fmt.Errorf("invalid %s value: %w", SourceDateEpochEnv, err)
+	}
+	return t, nil
+}
+
+// ParseSourceDateEpoch parses the given source date epoch, as *time.Time.
+// It returns an error if sourceDateEpoch is empty or not well-formatted.
+func ParseSourceDateEpoch(sourceDateEpoch string) (*time.Time, error) {
+	if sourceDateEpoch == "" {
+		return nil, fmt.Errorf("value is empty")
+	}
+	i64, err := strconv.ParseInt(sourceDateEpoch, 10, 64)
+	if err != nil {
+		return nil, fmt.Errorf("invalid value: %w", err)
+	}
+	unix := time.Unix(i64, 0).UTC()
+	return &unix, nil
+}
+
+// SetSourceDateEpoch sets the SOURCE_DATE_EPOCH env var.
+func SetSourceDateEpoch(tm time.Time) {
+	_ = os.Setenv(SourceDateEpochEnv, fmt.Sprintf("%d", tm.Unix()))
+}
+
+// UnsetSourceDateEpoch unsets the SOURCE_DATE_EPOCH env var.
+func UnsetSourceDateEpoch() {
+	_ = os.Unsetenv(SourceDateEpochEnv)
+}