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Dockerfile issue - Why is the binary dlv not being found - No such file or directory

I had a docker file that was working fine. However to remote debug it , I read that I needed to install dlv on it and then I need to run dlv and pass the parameter of the app I am trying to debug. So after installing dlv on it and attempting to run it. I get the error

exec /dlv: no such file or directory

This is the docker file

FROM golang:1.18-alpine AS builder

# Build Delve for debugging
RUN go install github.com/go-delve/delve/cmd/dlv@latest

# Create and change to the app directory.
WORKDIR /app
ENV CGO_ENABLED=0


# Retrieve application dependencies.
COPY go.* ./
RUN go mod download

# Copy local code to the container image.
COPY . ./


# Build the binary.
RUN go build -gcflags="all=-N -l" -o fooapp

# Use the official Debian slim image for a lean production container.
FROM debian:buster-slim

EXPOSE 8000 40000

RUN set -x && apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
    ca-certificates && \
    rm -rf /var/lib/apt/lists/*

# Copy the binary to the production image from the builder stage.
#COPY --from=builder /app/fooapp /app/fooapp #commented this out  

COPY --from=builder /go/bin/dlv /dlv

# Run dlv as pass fooapp as parameter
CMD ["/dlv", "--listen=:40000", "--headless=true", "--api-version=2", "--accept-multiclient", "exec", "/app/fooapp"]

The above results in exec /dlv: no such file or directory

I am not sure why this is happening. Being new to docker , I have tried different ways to debug it. I tried using dive to check and see if the image has dlv on it in the path /dlv and it does. I have also attached an image of it

enter image description here

Solution

  • You built dlv in alpine-based distro. dlv executable is linked against libc.musl:

    # ldd dlv 
        linux-vdso.so.1 (0x00007ffcd251d000)
        libc.musl-x86_64.so.1 => not found

    But then you switched to glibc-based image debian:buster-slim. That image doesn't have the required libraries.

    # find / -name libc.musl*                                        
<nothing found>

    That's why you can't execute dlv - the dynamic linker fails to find the proper lib.

    You need to build in glibc-based docker. For example, replace the first line

    FROM golang:bullseye AS builder

    BTW. After you build you need to run the container in the priviledged mode

    $ docker build . -t try-dlv
...
$ docker run --privileged --rm try-dlv
API server listening at: [::]:40000
2022-10-30T10:51:02Z warning layer=rpc Listening for remote connections (connections are not authenticated nor encrypted)

    In non-priviledged container dlv is not allowed to spawn a child process.

    $ docker run --rm try-dlv
API server listening at: [::]:40000
2022-10-30T10:55:46Z warning layer=rpc Listening for remote connections (connections are not authenticated nor encrypted)
could not launch process: fork/exec /app/fooapp: operation not permitted

    Really Minimal Image

    You use debian:buster-slim to minimize the image, it's size is 80 MB. But if you need a really small image, use busybox, it is only 4.86 MB overhead.

    FROM golang:bullseye AS builder

# Build Delve for debugging
RUN go install github.com/go-delve/delve/cmd/dlv@latest

# Create and change to the app directory.
WORKDIR /app
ENV CGO_ENABLED=0

# Retrieve application dependencies.
COPY go.* ./
RUN go mod download

# Copy local code to the container image.
COPY . ./

# Build the binary.
RUN go build -o fooapp .

# Download certificates
RUN set -x && apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
    ca-certificates 

# Use the official Debian slim image for a lean production container.
FROM busybox:glibc

EXPOSE 8000 40000

# Copy the binary to the production image from the builder stage.
COPY --from=builder /app/fooapp /app/fooapp 
# COPY --from=builder /app/ /app

COPY --from=builder /go/bin/dlv /dlv

COPY --from=builder /etc/ssl /etc/ssl

# Run dlv as pass fooapp as parameter
CMD ["/dlv", "--listen=:40000", "--headless=true", "--api-version=2", "--accept-multiclient", "exec", "/app/fooapp"]
# ENTRYPOINT ["/bin/sh"]

    The image size is 25 MB, of which 18 MB are from dlv and 2 MB are from Hello World application.

    While choosing the images care should be taken to have the same flavors of libc. golang:bullseye links against glibc. Hence, the minimal image must be glibc-based.

    But if you want a bit more comfort, use alpine with gcompat package installed. It is a reasonably rich linux with lots of external packages for just extra 6 MB compared to busybox.

    FROM golang:bullseye AS builder

# Build Delve for debugging
RUN go install github.com/go-delve/delve/cmd/dlv@latest

# Create and change to the app directory.
WORKDIR /app
ENV CGO_ENABLED=0

# Copy local code to the container image.
COPY . ./

# Retrieve application dependencies.
RUN go mod tidy

# Build the binary.
RUN go build -o fooapp .

# Use alpine lean production container.
# FROM busybox:glibc
FROM alpine:latest

# gcompat is the package to glibc-based apps
# ca-certificates contains trusted TLS CA certs
# bash is just for the comfort, I hate /bin/sh
RUN apk add gcompat ca-certificates bash

EXPOSE 8000 40000

# Copy the binary to the production image from the builder stage.
COPY --from=builder /app/fooapp /app/fooapp 
# COPY --from=builder /app/ /app

COPY --from=builder /go/bin/dlv /dlv

# Run dlv as pass fooapp as parameter
CMD ["/dlv", "--listen=:40000", "--headless=true", "--api-version=2", "--accept-multiclient", "exec", "/app/fooapp"]
# ENTRYPOINT ["/bin/bash"]