Unable to update/install curl (“the following packages have unmet dependencies”)

Question

I'm unable to update or install curl, due to a dependency mismatch problem. (I tried removing and reinstalling, thinking that might help, but it didn't.)

My curl attempt:

pi@RECOVERY:~ $ sudo apt-get install curl
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 curl : Depends: libcurl3 (= 7.52.1-5+deb9u1) but 7.38.0-4+deb8u5 is to be installed
E: Unable to correct problems, you have held broken packages.

And then an attempt at one of the dependencies:

pi@RECOVERY:~ $ sudo apt-get install libcurl3
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libcurl3 : Depends: libgssapi-krb5-2 (>= 1.14+dfsg) but 1.12.1+dfsg-19+deb8u2 is to be installed
            Depends: libnghttp2-14 (>= 1.12.0) but it is not installable
            Depends: libpsl5 (>= 0.13.0) but it is not installable
            Depends: libssh2-1 (>= 1.7.0) but 1.4.3-4.1+deb8u1 is to be installed
            Depends: libssl1.0.2 (>= 1.0.2d) but it is not installable
E: Unable to correct problems, you have held broken packages.

I also tried sudo apt-get update && sudo apt-get upgrade && sudo apt-get -f install, but that didn't work either. Neither did sudo apt-get purge.

I see that some of the sub-dependencies (e.g. libnghttp2-14, libpsl5, etc.) are shown as "not installable." Could this have something to do with it?

How can I get past this?

--EDIT--

As requested in the comments, here's my /etc/apt/sources.list, as well as the output from apt-cache policy curl:

sources.list

deb http://mirrordirector.raspbian.org/raspbian/ jessie main contrib non-free rpi
# Uncomment line below then 'apt-get update' to enable 'apt-get source'
#deb-src http://archive.raspbian.org/raspbian/ jessie main contrib non-free rpi
deb http://security.debian.org stable/updates main
deb-src http://security.debian.org/ stable/updates main

apt-cache

pi@RECOVERY:~ $ apt-cache policy curl
curl:
  Installed: (none)
  Candidate: 7.52.1-5+deb9u1
  Version table:
     7.52.1-5+deb9u1 0
        500 http://security.debian.org/ stable/updates/main armhf Packages
     7.38.0-4+deb8u6 0
        500 http://mirrordirector.raspbian.org/raspbian/ jessie/main armhf Packages
        500 http://archive.raspbian.org/raspbian/ jessie/main armhf Packages

Answer 1

Your sources.list mixes Raspbian Jessie, and Debian “stable” which is now Stretch (Debian 9); that’s why you’re running into this problem: installing curl tries to install the security update to curl from Debian 9, using dependencies from Raspbian Jessie, which doesn’t work.

To fix the problem, replace stable with jessie in sources.list:

deb http://security.debian.org jessie/updates main
deb-src http://security.debian.org jessie/updates main

Once that’s done, run apt update and then you should be able to install curl.

When following stable Debian releases, you should always use the release code-name rather than “stable” since the latter will suddenly change release when a new release is made.

Answer 2

If your Distro doesn't candidate versions of these dependencies you can add Debian repository to your /etc/apt/sources.list and install the dependencies by yourself

as suggested here

Make sure you have this line in /etc/apt/sources.list:

deb-src http://ftp.debian.org/debian/ stable main non-free contrib

APT provides a command "apt-get source" that looks in these deb-src lines (rather than plain binary deb lines) and downloads source packages. In this tutorial, you'll use "apt-src" which is a convenient wrapper around "apt-get source". Step 2

apt-get update

Step 3

sudo aptitude install apt-src

apt-src is a helper program that makes compiling source packages easy. It's not necessary, but it prevents you from having to type too many commands. Step 4

 apt-src -bi install $package

If you wanted to install 'libnghttp2-14' , run this:

apt-src -bi install libnghttp2-14 

The "b" stands for "build", the "i" stands for "install the resulting package", and the word "install" means "download the source for alpine as found in a Debian source line from sources.list". apt-src will "install" the source into the current directory, make sure you have all the required packages to build the package (a process called "satisfying the build dependencies"), build it, and install the resulting .debs.