progress: if not escaped, backslashes `\` not printed on the progress stream dockerfile steps

[profnandaa]

However, prints when escaped or put within "..":

Minimal repro dockerfile:

FROM mcr.microsoft.com/windows/nanoserver:ltsc2022

RUN echo C:\hello\world\path
RUN echo C:\\hello\\escaped\\path
RUN echo "C:\hello\quoted\path"

Result:

#5 [2/4] RUN echo C:helloworldpath
#5 1.359 C:\hello\world\path
#5 DONE 1.7s

#6 [3/4] RUN echo C:\hello\escaped\path
#6 1.734 C:\\hello\\escaped\\path
#6 DONE 2.1s

#7 [4/4] RUN echo "C:\hello\quoted\path"
#7 1.765 "C:\hello\quoted\path"
#7 DONE 2.1s

With classic docker build:

Result:

Step 2/4 : RUN echo C:\hello\world\path
 ---> Running in 94d37a87e491
C:\hello\world\path
 ---> Removed intermediate container 94d37a87e491
 ---> b098d8792b7c
Step 3/4 : RUN echo C:\\hello\\escaped\\path
 ---> Running in d2fcea089fb1
C:\\hello\\escaped\\path
 ---> Removed intermediate container d2fcea089fb1
 ---> e6c9869a97f8
Step 4/4 : RUN echo "C:\hello\quoted\path"
 ---> Running in 2e2c603aebe2
"C:\hello\quoted\path"
 ---> Removed intermediate container 2e2c603aebe2
 ---> 56ba89eb540e

[ardrabczyk]

It can also be reproduced on Linux:

$ cat Dockerfile
FROM alpine

RUN echo mkdir C:\another\sample
RUN echo mkdir C:\\another\\sample
$ docker build -t backslash-test --no-cache .
[+] Building 0.8s (7/7) FINISHED                                                                                                                                                                     docker:default
 => [internal] load .dockerignore                                                                                                                                                                              0.0s
 => => transferring context: 2B                                                                                                                                                                                0.0s
 => [internal] load build definition from Dockerfile                                                                                                                                                           0.0s
 => => transferring dockerfile: 118B                                                                                                                                                                           0.0s
 => [internal] load metadata for docker.io/library/alpine:latest                                                                                                                                               0.2s
 => CACHED [1/3] FROM docker.io/library/alpine@sha256:0a4eaa0eecf5f8c050e5bba433f58c052be7587ee8af3e8b3910ef9ab5fbe9f5                                                                                         0.0s
 => [2/3] RUN echo mkdir C:anothersample                                                                                                                                                                       0.2s
 => [3/3] RUN echo mkdir C:\another\sample                                                                                                                                                                     0.2s
 => exporting to image                                                                                                                                                                                         0.1s
 => => exporting layers                                                                                                                                                                                        0.1s
 => => writing image sha256:cfbd890f4a908e48975d6c462a8b7b447effe943208c64625f2e0601942af236                                                                                                                   0.0s
 => => naming to docker.io/library/backslash-test                                                                                                                                                              0.0s

[profnandaa]

@ardrabczyk -- good catch, thanks! Updated the title and description.

[profnandaa]

I have narrowed down to this, frontend/dockerfile/dockerfile2llb/convert.go:2055

> github.com/moby/buildkit/frontend/dockerfile/dockerfile2llb.processCmdEnv() C:/dev/buildkit/frontend/dockerfile/dockerfile2llb/convert.go:2057 (PC: 0x238dec1)

  2055: func processCmdEnv(shlex *shell.Lex, cmd string, env shell.EnvGetter) string {
  2056:         w, _, err := shlex.ProcessWord(cmd, env)
=>2057:         if err != nil {
  2058:                 return cmd
  2059:         }
  2060:         return w
  2061: }
  2062:
(dlv) p cmd
"RUN echo C:\\hello\\world\\path"
(dlv) p w
"RUN echo C:helloworldpath"
~~~~~~~~~~~^^--- backslashes being stripped off by the lexer.
(dlv)

I'm not really conversant with this lexer, will appreciate any tips? @tonistiigi