Big bang

.gitmodules

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+[submodule "lib/g.bash"]
+	path = lib/g.bash
+	url = https://code.garrettmills.dev/garrettmills/g.bash

.idea/.gitignore

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+# Default ignored files
+/shelf/
+/workspace.xml
+# Editor-based HTTP Client requests
+/httpRequests/
+# Datasource local storage ignored files
+/dataSources/
+/dataSources.local.xml

.idea/docker-isolate.iml

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+<?xml version="1.0" encoding="UTF-8"?>
+<module type="JAVA_MODULE" version="4">
+  <component name="NewModuleRootManager" inherit-compiler-output="true">
+    <exclude-output />
+    <content url="file://$MODULE_DIR$" />
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>

.idea/misc.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_17" default="true" project-jdk-name="17" project-jdk-type="JavaSDK">
+    <output url="file://$PROJECT_DIR$/out" />
+  </component>
+</project>

.idea/modules.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/.idea/docker-isolate.iml" filepath="$PROJECT_DIR$/.idea/docker-isolate.iml" />
+    </modules>
+  </component>
+</project>

.idea/vcs.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>

Dockerfile

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+FROM ubuntu:22.04
+
+COPY README.md /README.md
+
+RUN echo "Fubar!" > /fubar

README.md

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+Goal:
+
+- Specify a base image name
+- Specify a final image name
+- Identify the last layer of the base image
+- Export the intermediate layers that the final image adds to the base image
+- Reimport those as a single layer

docker-isolate.bash

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+#!/bin/bash -e
+
+SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
+. "${SCRIPT_DIR}/lib/g.bash/src/g.bash"
+
+g::source src/setup
+g::source src/isolate
+
+g::app dlt
+
+g::app::command isolate "Build a Docker image with only the changed layers between two images"
+g::app::command::arg from_image "The base (starting) image, whose layers will be excluded"
+g::app::command::arg to_image "The final (ending) image, whose additional layers will be isolated"
+g::app::command::flag output= "Path for the output archive"
+g::app::command::flag as= "Reimport the isolated filesystem as the specified image name"
+g::app::command::flag no-base "When specified with --as, the isolated filesystem will NOT be applied to the base image"
+
+g::app::invoke "$@"

lib/g.bash

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+Subproject commit c5141fd19e96066f154937f2d401d76560660d36

lib/jq/README.md

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+https://github.com/jqlang/jq
+
+`jq`: Command-line JSON processor
+Binary version 1.7.1
+Licensed MIT

src/dkr_functions.bash

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+#!/bin/bash -e
+
+function dkr::save_and_extract() {
+    # image name = $1
+    # file name = $2
+    g::verbose "Extracting image $1 as $2"
+    docker save "$1" -o "$2.tar"
+    mkdir "$2"
+    cd "$2"
+    tar xf "../$2.tar"
+    cd - > /dev/null
+    rm -f "$2.tar"
+}
+
+function dkr::import_fs_as_layer() {
+    # tar path = $1
+    # base image = $2
+    # final image = $3
+    docker image import "$1" docker-isolate-flat-temp:latest > /dev/null
+    echo "FROM docker-isolate-flat-temp:latest as isolated" > Dockerfile
+    echo "FROM $2 as from_image" >> Dockerfile
+    echo "COPY --from=isolated / /" >> Dockerfile
+    docker image build -f Dockerfile -t "$3" . > /dev/null
+    docker image rm -f docker-isolate-flat-temp:latest > /dev/null
+    rm -f Dockerfile
+}

src/isolate.bash

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+#!/bin/bash -e
+
+g::source dkr_functions
+
+function app::dlt::isolate() {
+    local args="$1"
+
+    from_image="$(g::arg "$args" from_image)"
+    to_image="$(g::arg "$args" to_image)"
+    g::info "Isolating changes from $from_image to $to_image"
+
+    # Resolve the output path for the tarball since we're going to be changing directories
+    output_path="$(g::flag "$args" output)"
+    if [ -z "$output_path" ]; then
+        output_path="$(pwd)/isolated.tar"
+    fi
+    output_path="$(g::path::resolve "$output_path")"
+    g::verbose "Output path: $output_path"
+
+    g::path::cd "$(g::path::tmpdir)"
+    g::verbose "Working dir: $(pwd)"
+
+    # Extract both the base and final image in container layer format
+    dkr::save_and_extract "$to_image" to_image
+    dkr::save_and_extract "$from_image" from_image
+
+    # Load arrays of BOTH the raw layer tarballs and the SHA sums of the layers in the final image
+    to_json_path="$(jq -r '.[0].Config' < to_image/manifest.json)"
+    g::verbose "Found config for to_image: ${to_json_path}"
+
+    readarray -t to_raw_layers <<< "$(jq -r '.[0].Layers[]' < to_image/manifest.json)"
+    readarray -t to_sha_layers <<< "$(jq -r '.rootfs.diff_ids[]' < "to_image/${to_json_path}")"
+
+    # Load an array of the SHA sums of the layers in the base image
+    from_json_path="$(jq -r '.[0].Config' < from_image/manifest.json)"
+    g::verbose "Found config for from_image: ${from_json_path}"
+
+    readarray -t from_sha_layers <<< "$(jq -r '.rootfs.diff_ids[]' < "from_image/${from_json_path}")"
+
+    mkdir isolated
+    cd isolated
+
+    # Extract the isolated layers
+    for ((i = 0 ; i < "${#to_raw_layers[@]}" ; i++)); do
+        raw_layer="${to_raw_layers[$i]}"
+        sha_layer="${to_sha_layers[$i]}"
+
+        # If the SHA sum of this layer appears in the base image, skip it
+        if g::arr::includes "$sha_layer" "${from_sha_layers[@]}"; then
+            g::debug "Skipping layer from base image: $sha_layer"
+            continue
+        fi
+
+        # Extract the layer into the isolated directory
+        g::info "Unpacking layer: $sha_layer -> $raw_layer"
+        tar xf "../to_image/${raw_layer}"
+    done
+
+    # Create a tarball w/ the isolated files (special find magic to avoid the leading ./ on paths)
+    g::info 'Creating archive'
+    cd ..
+    find isolated/ \( -type f -o -type d \) -printf "%P\n" | tar cf isolated.tar -C isolated/ -T -
+
+    # Do a bit of cleanup
+    rm -rf to_image from_image isolated
+
+    # If the user asked for the isolated filesystem to be re-imported as a docker image, do that
+    as_image="$(g::flag "$args" as)"
+    if [ -n "$as_image" ]; then
+        if g::flag::has "$args" no-base; then
+            g::info "Creating image $as_image without base layers"
+            docker image import isolated.tar "$as_image" > /dev/null
+        else
+            g::info "Creating image $as_image on top of $from_image"
+            dkr::import_fs_as_layer isolated.tar "$from_image" "$as_image"
+        fi
+    fi
+
+    # Create the output archive and exit
+    g::verbose "Moving archive to final destination"
+    mv isolated.tar "$output_path"
+}

src/setup.bash

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+#!/bin/bash -e
+
+SRC_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
+export PATH="${SRC_DIR}/lib/jq:${PATH}"
+
+g::log::setLevel info
+