dnf –assumeno & PostgreSQL libs: Why Skipping Dependencies Fails

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Summary

A developer attempted to preview a system update using the dnf update --assumeno flag. The transaction report indicated that several packages required postgresql-libs. The developer’s core concern was whether manually bypassing or disabling these dependencies during the actual execution would result in package conflicts or if the package manager would automatically re-inject them.

The fundamental misunderstanding lies in the distinction between a transaction preview and dependency enforcement.

Root Cause

The issue stems from a confusion between dependency reporting and dependency skipping.

  • The --assumeno behavior: This flag is strictly a dry-run mechanism. It tells the package manager to simulate the entire dependency tree resolution and then immediately halt before any changes are applied.
  • Dependency Reporting: When dnf shows a dependency like postgresql-libs, it is not proposing a change to remove it; it is informing the user that the proposed transaction requires that package to maintain system integrity.
  • The Conflict Myth: You cannot “disable” a dependency during a standard dnf update without using specialized flags like --skip-broken or --exclude. If you attempt to install a package while explicitly preventing its required dependencies from being installed, the transaction will fail because the dependency solver cannot reach a “satisfied” state.

Why This Happens in Real Systems

In production environments, dependency trees become increasingly complex due to:

  • Shared Libraries: Many non-database applications (like web servers, monitoring agents, or CLI tools) link against postgresql-libs for specific data types or communication protocols.
  • Transitive Dependencies: Package A depends on Package B, which depends on postgresql-libs. Even if you aren’t running a database, the library is a prerequisite for the chain to function.
  • Version Pinning: An update to a system library might force an update to the PostgreSQL library to ensure ABI (Application Binary Interface) compatibility.

Real-World Impact

If an engineer attempts to force an update while ignoring dependency requirements (e.g., via --exclude or manual RPM manipulation):

  • Broken Binaries: Applications relying on the specific version of the library will encounter error while loading shared libraries: libpq.so.x: cannot open shared object file.
  • System Instability: If core system utilities share these libraries, a partial update can lead to a non-bootable state or a broken shell.
  • Dependency Hell: Attempting to “fix” a broken state by force-installing packages often leads to a recursive loop of conflicts that requires manual database reconstruction.

Example or Code (if necessary and relevant)

To see how dnf identifies the dependency chain without actually changing anything, use the repoquery tool:

dnf repoquery --requires --resolve postgresql-libs
dnf repoquery --whatrequires postgresql-libs

How Senior Engineers Fix It

Senior engineers do not try to “disable” dependencies; they resolve the underlying requirement.

  • Analyze the Chain: Use dnf repoquery --whatrequires <package> to understand exactly which applications will break if that dependency is removed.
  • Controlled Updates: If a specific package version is causing issues, use a module stream (in DNF/Modular systems) or an exclude flag specifically for the problematic package, rather than trying to bypass the dependency itself.
  • Staging Environments: Never run a major dnf update on a production node without first verifying the transaction on a clone of the production environment.
  • Atomic Transactions: Use filesystem snapshots (like Btrfs or LVM) before performing updates so the system can be rolled back instantly if the dependency resolution leads to an unstable state.

Why Juniors Miss It

  • Misinterpreting Output: Juniors often view dnf output as a “list of things to be deleted” or “things that are wrong,” rather than a mathematical proof of requirements.
  • Fear of Change: There is often a hesitation to let the package manager install “extra” things (like the PostgreSQL libs), leading to the mistaken belief that they can be “skipped” to keep the system “clean.”
  • Focus on the Symptom: Instead of asking “Why does this package need PostgreSQL?”, they ask “How do I stop this package from asking for PostgreSQL?”

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