Fix control-plane join failing with empty certificate key

[alicefr]

The upload-certs command was piped through 2>/dev/null | tail -1, which suppressed errors and masked the exit code. When kubeadm failed, we got an empty string instead of an error.

Remove the pipe and stderr suppression so failures propagate properly. Extract the certificate key by matching the 64-char hex string from the output instead of blindly taking the last line.

Summary by Sourcery

Ensure control-plane join correctly obtains the kubeadm certificate key by using the full upload-certs output and parsing the hex key instead of relying on a piped tail command.

Bug Fixes:

• Fix control-plane joins failing due to empty certificate keys when kubeadm upload-certs reports an error that was previously suppressed.

Enhancements:

• Improve robustness of certificate key extraction by scanning upload-certs output for a 64-character hexadecimal key and surfacing detailed errors when not found.

[sourcery-ai]

Reviewer's Guide

Adjusts the control-plane join flow to preserve kubeadm error propagation and robustly extract the certificate key from upload-certs output, avoiding failures due to empty keys.

Sequence diagram for updated control-plane join certificate key extraction

sequenceDiagram
    participant Cluster
    participant cpSSHClient
    participant kubeadm

    Cluster->>Cluster: generateJoinCommand(ctx, cpSSHClient, true)
    Cluster->>cpSSHClient: Exec(ctx, sudo kubeadm init phase upload-certs --upload-certs)
    cpSSHClient->>kubeadm: run upload-certs
    kubeadm-->>cpSSHClient: certKeyOutput (logs on stderr, key on stdout)
    cpSSHClient-->>Cluster: certKeyOutput

    loop for each line in certKeyOutput
        Cluster->>Cluster: strings.TrimSpace(line)
        alt len(line) == 64 and isHex(line)
            Cluster->>Cluster: set certificateKey
        end
    end

    alt certificateKey == ""
        Cluster->>Cluster: fmt.Errorf(certificate key not found in upload-certs output)
        Cluster-->>Cluster: generateJoinCommand returns error
    else certificateKey != ""
        Cluster->>Cluster: logger.Infof(Certificate key: %s, certificateKey)
        Cluster-->>Cluster: generateJoinCommand returns join command
    end

Loading

File-Level Changes

Change	Details	Files
Stop suppressing kubeadm upload-certs errors and parsing the certificate key via a fragile tail-based pipeline; instead, run the command directly and parse a 64-character hex key from its output.	Remove the shell pipeline `2>/dev/null	tail -1from the kubeadm upload-certs invocation so stderr is not discarded and the exit code is preserved.</li><li>Capture the full stdout ofkubeadm init phase upload-certs --upload-certs` and scan each line for a 64-character hex string, treating that as the certificate key. Improve error handling by failing with a descriptive error message that includes the full upload-certs output when no valid certificate key is found.
Introduce a small utility to validate hexadecimal strings used when extracting the certificate key.	Add an isHex helper function that returns true only if a string consists solely of 0–9, a–f, or A–F characters. Use the isHex helper when scanning upload-certs output to avoid misinterpreting unrelated log lines as the certificate key.	internal/cluster/join.go

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[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• When scanning for the certificate key, consider breaking out of the loop as soon as a valid 64-char hex string is found instead of overwriting certificateKey on each match, to avoid unintentionally preferring a later hex-looking token if kubeadm ever prints more than one.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- When scanning for the certificate key, consider breaking out of the loop as soon as a valid 64-char hex string is found instead of overwriting `certificateKey` on each match, to avoid unintentionally preferring a later hex-looking token if kubeadm ever prints more than one.

## Individual Comments

### Comment 1
<location path="internal/cluster/join.go" line_range="167-168" />
<code_context>
+				certificateKey = line
+			}
+		}
 		if certificateKey == "" {
-			return "", fmt.Errorf("certificate key is empty")
+			return "", fmt.Errorf("certificate key not found in upload-certs output: %s", certKeyOutput)
 		}

</code_context>
<issue_to_address>
**🚨 issue (security):** Avoid returning the full upload-certs output (including the key) in the error message

Including `certKeyOutput` here will log and propagate the certificate key, which is sensitive and should never be exposed. Instead, either omit the raw output from the error or redact the key (e.g., log only a short, non-sensitive snippet or use a structured message without the full value).
</issue_to_address>

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[sourcery-ai]

🚨 issue (security): Avoid returning the full upload-certs output (including the key) in the error message

Including certKeyOutput here will log and propagate the certificate key, which is sensitive and should never be exposed. Instead, either omit the raw output from the error or redact the key (e.g., log only a short, non-sensitive snippet or use a structured message without the full value).