Tweak our handling of variant values, again

Ensure that when not invoked with --platform, we use the default
platform, and that when invoked with --platform, we use the platform
information from the base image that we ended up using for the build,
with no accidental mixing of the two.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

config.go

@@ -146,18 +146,9 @@ func (b *Builder) fixupConfig(sys *types.SystemContext) {
 	if b.Architecture() == "" {
 		if sys != nil && sys.ArchitectureChoice != "" {
 			b.SetArchitecture(sys.ArchitectureChoice)
-		} else {
-			b.SetArchitecture(currentPlatformSpecification.Architecture)
-		}
-		// in case the arch string we started with was shorthand for a known arch+variant pair, normalize it
-		ps := internalUtil.NormalizePlatform(ociv1.Platform{OS: b.OS(), Architecture: b.Architecture(), Variant: b.Variant()})
-		b.SetArchitecture(ps.Architecture)
-		b.SetVariant(ps.Variant)
-	}
-	if b.Variant() == "" {
-		if sys != nil && sys.VariantChoice != "" {
 			b.SetVariant(sys.VariantChoice)
 		} else {
+			b.SetArchitecture(currentPlatformSpecification.Architecture)
 			b.SetVariant(currentPlatformSpecification.Variant)
 		}
 		// in case the arch string we started with was shorthand for a known arch+variant pair, normalize it

tests/platforms.bats

@@ -0,0 +1,75 @@
+#!/usr/bin/env bats
+
+load helpers
+
+# read the platform information from the configuration of the main image for
+# the oci layout in $1
+read_oci_layout_platform() {
+  run jq -r '.manifests[0].digest' "$1"/index.json
+  assert $status -eq 0
+  local alg="${output%%:*}"
+  local hex="${output##*:}"
+  run jq -r '.config.digest' "$1"/blobs/"$alg"/"$hex"
+  assert $status -eq 0
+  alg="${output%%:*}"
+  hex="${output##*:}"
+  run jq -r '.os' "$1"/blobs/"$alg"/"$hex"
+  assert $status -eq 0
+  local os="$output"
+  run jq -r '.architecture' "$1"/blobs/"$alg"/"$hex"
+  assert $status -eq 0
+  local arch="$output"
+  run jq -r '.variant' "$1"/blobs/"$alg"/"$hex"
+  assert $status -eq 0
+  local variant="$output"
+  if test "$variant" = null ; then
+    variant=
+  fi
+  echo "$os"/"$arch""${variant:+/$variant}"
+}
+
+@test "implicit-and-explicit-platforms" {
+  _prefetch busybox
+  local context="$TEST_SCRATCH_DIR"/context
+  mkdir -p "$context"
+  cat > "$context"/Dockerfile.scratch << EOF
+  FROM scratch
+  COPY . .
+EOF
+  cat > "$context"/Dockerfile.base << EOF
+  FROM busybox
+EOF
+  cat > "$context"/Dockerfile.derived << EOF
+  FROM busybox
+  COPY . .
+EOF
+  run_buildah version --json
+  run jq -r .buildPlatform <<< "$output"
+  assert $status -eq 0
+  local buildplatform="$output"
+
+  # these should either get the default determined at runtime, or the value passed
+  for platform in "" linux/amd64 linux/arm64 linux/arm64/v8 ; do
+    local arch="${platform##*/}"
+    run_buildah build --layers --no-cache -t oci:"$TEST_SCRATCH_DIR"/scratch-"${arch:-default}" ${platform:+--platform "$platform"} -f "$context"/Dockerfile.scratch "$context"
+    run read_oci_layout_platform "$TEST_SCRATCH_DIR"/scratch-"${arch:-default}"
+    assert $status -eq 0
+    assert "$output" = "${platform:-${buildplatform}}" "for build based on scratch for ${platform:-default platform}"
+  done
+
+  # these should inherit the values from the base image that we used for the given platform
+  for platform in "" linux/amd64 linux/arm64 linux/arm64/v8 ; do
+    arch="${platform##*/}"
+    for base in base derived ; do
+      run_buildah build --layers --no-cache -t "$base"-"${arch:-default}" ${platform:+--platform "$platform"} -f "$context"/Dockerfile."$base" "$context"
+      run_buildah push "$base"-"${arch:-default}" oci:"$TEST_SCRATCH_DIR"/"$base"-"${arch:-default}"
+    done
+    run read_oci_layout_platform "$TEST_SCRATCH_DIR"/base-"${arch:-default}"
+    assert $status -eq 0
+    baseplatform="$output"
+    run read_oci_layout_platform "$TEST_SCRATCH_DIR"/derived-"${arch:-default}"
+    assert $status -eq 0
+    derivedplatform="$output"
+    assert "$baseplatform" = "$derivedplatform" "for build based for ${platform:-default platform}"
+  done
+}