Reading multiple documents from a YAML file and marshaling them with go yaml v3

I'm trying to use gopkg.in/yaml.v3 to read a file containing
multiple YAML documents, and then marshal each individual document
back into YAML. My sample input looks like:

name: doc1
---
name: doc2
---
name: doc3

I'm able to unmarshal the file just fine, but I run into unexpected
errors when trying to marshal the individual documents. My code looks
like:

package main

import (
	"errors"
	"fmt"
	"io"

	"os"

	"gopkg.in/yaml.v3"
)

func main() {
	reader := io.Reader(os.Stdin)
	dec := yaml.NewDecoder(reader)

	for {
		var node yaml.Node
		err := dec.Decode(&node)
		if errors.Is(err, io.EOF) {
			break
		}
		if err != nil {
			panic(err)
		}

		content, err := yaml.Marshal(node)
		if err != nil {
			panic(err)
		}

		fmt.Printf("Found a doc\n\n%s\n", content)
	}
}

Running this code with the sample input results in:

panic: yaml: expected SCALAR, SEQUENCE-START, MAPPING-START, or ALIAS, but got document start

goroutine 1 [running]:
main.main()
        /home/lars/projects/operate-first/halberd/main.go:31 +0x451

The code seems to be parsing the document correctly; if I replace it
with:

package main

import (
	"errors"
	"fmt"
	"io"

	"os"

	"gopkg.in/yaml.v3"
)

func main() {
	reader := io.Reader(os.Stdin)
	dec := yaml.NewDecoder(reader)

	for {
		var node yaml.Node
		err := dec.Decode(&node)
		if errors.Is(err, io.EOF) {
			break
		}
		if err != nil {
			panic(err)
		}

		fmt.Printf("Found a doc\n")
	}
}

I end up with what I expect:

Found a doc
Found a doc
Found a doc

The problem only crops up when marshaling. Am I correctly using the v3 api?

There was a small mistake in your code.
It should be content, err := yaml.Marshal(&node). You missed the &.
This works,

package main

import (
    "errors"
    "fmt"
    "io"

    "os"

    "gopkg.in/yaml.v3"
)

func main() {
    reader := io.Reader(os.Stdin)
    dec := yaml.NewDecoder(reader)

    for {
        var node yaml.Node
        err := dec.Decode(&node)
        if errors.Is(err, io.EOF) {
            break
        }
        if err != nil {
            panic(err)
        }

        content, err := yaml.Marshal(&node)
        if err != nil {
            panic(err)
        }

        fmt.Printf("Found a doc\n\n%s\n", content)
    }
}

The docs for yaml.Marshal says,
> Marshal serializes the value provided into a YAML document. The structure of the generated document will reflect the structure of the value itself. Maps and pointers (to struct, string, int, etc) are accepted as the in value.

Source: https://pkg.go.dev/gopkg.in/yaml.v2#Marshal

Since you are trying to marshal yaml.Node which is a struct, you must pass the pointer to the struct and not the value itself, as the docs say.