rpk: make set command to only write desired value

there was a bug that made rpk redpanda config set
to write unset defaults to the config file even
if the user didn't request it

src/go/rpk/pkg/cli/cmd/redpanda/config.go

@@ -67,6 +67,7 @@ partial json/yaml config objects:
 			p := config.ParamsFromCommand(cmd)
 			cfg, err := p.Load(fs)
 			out.MaybeDie(err, "unable to load config: %v", err)
+			cfg = cfg.FileOrDefaults()
 
 			if format == "single" {
 				fmt.Println("'--format single' is deprecated, either remove it or use yaml/json")

src/go/rpk/pkg/cli/cmd/redpanda/config_test.go

@@ -184,3 +184,151 @@ func TestInitNode(t *testing.T) {
 		})
 	}
 }
+
+// This is a top level command test, individual cases for set are
+// tested in 'rpk/pkg/config/config_test.go'.
+func TestSetCommand(t *testing.T) {
+	for _, test := range []struct {
+		name    string
+		cfgFile string
+		exp     string
+		args    []string
+	}{
+		{
+			name: "set without config file on disk",
+			exp: `config_file: /etc/redpanda/redpanda.yaml
+redpanda:
+    data_directory: /var/lib/redpanda/data
+    node_id: 0
+    rack: redpanda-rack
+    seed_servers: []
+    rpc_server:
+        address: 0.0.0.0
+        port: 33145
+    kafka_api:
+        - address: 0.0.0.0
+          port: 9092
+    admin:
+        - address: 0.0.0.0
+          port: 9644
+    developer_mode: true
+rpk:
+    enable_usage_stats: false
+    tune_network: false
+    tune_disk_scheduler: false
+    tune_disk_nomerges: false
+    tune_disk_write_cache: false
+    tune_disk_irq: false
+    tune_fstrim: false
+    tune_cpu: false
+    tune_aio_events: false
+    tune_clocksource: false
+    tune_swappiness: false
+    tune_transparent_hugepages: false
+    enable_memory_locking: false
+    tune_coredump: false
+    coredump_dir: /var/lib/redpanda/coredump
+    tune_ballast_file: false
+    overprovisioned: false
+pandaproxy: {}
+schema_registry: {}
+`,
+			args: []string{"redpanda.rack", "redpanda-rack"},
+		},
+		{
+			name: "set with loaded config",
+			cfgFile: `config_file: /etc/redpanda/redpanda.yaml
+redpanda:
+    data_directory: ""
+    node_id: 0
+    rack: redpanda-rack
+    seed_servers: []
+    rpc_server:
+        address: 0.0.0.0
+        port: 33145
+    kafka_api:
+        - address: 0.0.0.0
+          port: 9092
+    admin:
+        - address: 0.0.0.0
+          port: 9644
+    developer_mode: true
+rpk:
+    enable_usage_stats: false
+    tune_network: false
+    tune_disk_scheduler: false
+    tune_disk_nomerges: false
+    tune_disk_write_cache: false
+    tune_disk_irq: false
+    tune_fstrim: false
+    tune_cpu: false
+    tune_aio_events: false
+    tune_clocksource: false
+    tune_swappiness: false
+    tune_transparent_hugepages: false
+    enable_memory_locking: false
+    tune_coredump: false
+    tune_ballast_file: false
+    overprovisioned: false
+`,
+			exp: `config_file: /etc/redpanda/redpanda.yaml
+redpanda:
+    node_id: 0
+    rack: redpanda-rack
+    seed_servers: []
+    rpc_server:
+        address: 0.0.0.0
+        port: 33145
+    kafka_api:
+        - address: 0.0.0.0
+          port: 9092
+    admin:
+        - address: 0.0.0.0
+          port: 9644
+    developer_mode: true
+rpk:
+    enable_usage_stats: true
+    tune_network: false
+    tune_disk_scheduler: false
+    tune_disk_nomerges: false
+    tune_disk_write_cache: false
+    tune_disk_irq: false
+    tune_fstrim: false
+    tune_cpu: false
+    tune_aio_events: false
+    tune_clocksource: false
+    tune_swappiness: false
+    tune_transparent_hugepages: false
+    enable_memory_locking: false
+    tune_coredump: false
+    tune_ballast_file: false
+    overprovisioned: false
+`,
+			args: []string{"rpk.enable_usage_stats", "true"},
+		},
+	} {
+		fs := afero.NewMemMapFs()
+
+		// We create a config file in default redpanda location
+		if test.cfgFile != "" {
+			err := afero.WriteFile(fs, "/etc/redpanda/redpanda.yaml", []byte(test.cfgFile), 0o644)
+			if err != nil {
+				t.Errorf("unexpected failure writing passed config file: %v", err)
+			}
+		}
+
+		c := set(fs)
+		c.SetArgs(test.args)
+		err := c.Execute()
+		if err != nil {
+			t.Errorf("error during command execution: %v", err)
+		}
+
+		// Read back from that default location and compare.
+		file, err := afero.ReadFile(fs, "/etc/redpanda/redpanda.yaml")
+		if err != nil {
+			t.Errorf("unexpected failure reading config file: %v", err)
+		}
+		require.Equal(t, test.exp, string(file))
+	}
+}

src/go/rpk/pkg/config/config.go

@@ -143,6 +143,21 @@ func AvailableModes() []string {
 	}
 }
 
+// FileOrDefaults return the configuration as read from the file or
+// the default configuration if there is no file loaded.
+func (c *Config) FileOrDefaults() *Config {
+	if c.File() != nil {
+		cfg := c.File()
+		cfg.loadedPath = c.loadedPath
+		cfg.ConfigFile = c.ConfigFile
+		return cfg
+	} else {
+		cfg := Default()
+		cfg.ConfigFile = c.ConfigFile // preserve ConfigFile property.
+		return cfg                    // no file, write the defaults
+	}
+}
+
 // Check checks if the redpanda and rpk configuration is valid before running
 // the tuners. See: redpanda_checkers.
 func (c *Config) Check() (bool, []error) {