Display a warning when ~ is used in a COPY destination (#3540)

Got some feedback about how `~` is expected to expand to a home
directory.
However this is not something that Earthly currently supports (see #1789
for more info).
This PR will display a warning if a `~` is used to make it clear to the
user.

CHANGELOG.md

@@ -4,6 +4,9 @@ All notable changes to [Earthly](https://github.com/earthly/earthly) will be doc
 
 ## Unreleased
 
+### Added
+- A warning when a `COPY` destination includes a tilde (~). Related to [#1789](https://github.com/earthly/earthly/issues/1789).
+
 ## v0.7.22 - 2023-11-27
 
 ### Added

earthfile2llb/interpreter.go

@@ -6,6 +6,7 @@ import (
 	"net"
 	"os"
 	"path"
+	"slices"
 	"strconv"
 	"strings"
 
@@ -969,6 +970,12 @@ func (i *Interpreter) handleCopy(ctx context.Context, cmd spec.Command) error {
 	if !allClassical && !allArtifacts {
 		return i.errorf(cmd.SourceLocation, "combining artifacts and build context arguments in a single COPY command is not allowed: %v", srcs)
 	}
+
+	if slices.ContainsFunc(strings.Split(dest, "/"), func(s string) bool {
+		return s == "~" || strings.HasPrefix(s, "~")
+	}) {
+		i.console.Warnf(`destination path %q contains a "~" which does not expand to a home directory`, dest)
+	}
 	if allArtifacts {
 		if dest == "" || dest == "." || len(srcs) > 1 {
 			dest += string("/") // TODO needs to be the containers platform, not the earthly hosts platform. For now, this is always Linux.

tests/Earthfile

@@ -26,6 +26,7 @@ ga-no-qemu-group1:
     BUILD +privileged-test
     BUILD +copy-test
     BUILD +copy-test-verbose-output
+    BUILD +copy-tilde-test
     BUILD +copy-keep-own-test
     BUILD +git-clone-test
     BUILD +builtin-args-invalid-default-test
@@ -227,7 +228,7 @@ copy-test:
         echo "1" > in/sub/1/file && \
         echo "2" > in/sub/2/file && \
         echo "sub" > in/sub/file
-    DO +RUN_EARTHLY --earthfile=copy.earth
+    DO +RUN_EARTHLY --earthfile=copy.earth --output_does_not_contain="which does not expand to a home directory"
 
 copy-test-verbose-output:
     RUN mkdir -p subdir/a.txt && \
@@ -264,6 +265,15 @@ copy-keep-own-test:
     DO +RUN_EARTHLY --target=+test-known-user --earthfile=copy-keep-own.earth
     DO +RUN_EARTHLY --target=+test-unknown-user --earthfile=copy-keep-own.earth
 
+copy-tilde-test:
+    RUN touch in
+    DO +RUN_EARTHLY --target=+copy-tilde-destination --earthfile=copy-tilde.earth --output_contains='destination path \"~/.\" contains a \"~\" which does not expand to a home directory'
+    DO +RUN_EARTHLY --target=+copy-tilde-in-destination --earthfile=copy-tilde.earth --output_contains='destination path \"/some/dir/~/.\" contains a \"~\" which does not expand to a home directory'
+    DO +RUN_EARTHLY --target=+copy-tilde-in-destination-prefix --earthfile=copy-tilde.earth --output_contains='destination path \"~some/dir.\" contains a \"~\" which does not expand to a home directory'
+    DO +RUN_EARTHLY --target=+copy-tilde-arg-in-destination --earthfile=copy-tilde.earth --output_contains='destination path \"/some/dir/~/.\" contains a \"~\" which does not expand to a home directory'
+    DO +RUN_EARTHLY --target=+copy-tilde-artifact --earthfile=copy-tilde.earth --output_contains='destination path \"/some/dir/~/.\" contains a \"~\" which does not expand to a home directory'
+    DO +RUN_EARTHLY --target=+copy-tilde-in-destination-not-prefix --earthfile=copy-tilde.earth --output_does_not_contain="which does not expand to a home directory"
+
 cache-test:
     # Test that a file can be passed between runs through the mounted cache.
     DO +RUN_EARTHLY --earthfile=cache1.earth --target=+test-pass-file --use_tmpfs=false

tests/copy-tilde.earth

@@ -0,0 +1,39 @@
+VERSION 0.7
+
+FROM alpine:3.18
+WORKDIR /test
+
+artifact:
+    COPY --dir in in
+    SAVE ARTIFACT in
+
+copy-tilde-destination:
+    RUN echo destination is tilde
+    COPY in ~/.
+
+copy-tilde-in-destination:
+    RUN echo destination contains tilde
+    RUN mkdir -p /some/dir
+    COPY in /some/dir/~/.
+
+copy-tilde-in-destination-prefix:
+    RUN echo destination contains tilde in prefix
+    RUN mkdir -p ~some/dir
+    COPY in ~some/dir.
+
+copy-tilde-arg-in-destination:
+    RUN echo destination arg contains tilde
+    RUN mkdir -p /some/dir
+    ARG dest=/some/dir/~/.
+    COPY in $dest
+
+copy-tilde-artifact:
+    RUN echo destination contains tilde for artifact
+    RUN mkdir -p /some/dir
+    ARG dest=/some/dir/~/.
+    COPY +artifact/in /some/dir/~/.
+
+copy-tilde-in-destination-not-prefix:
+    RUN echo destination contains a tilde but that should not trigger a warning
+    RUN mkdir -p some/di~r
+    COPY in some/di~r.