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while read -r NAME METHOD URL TAG ; do
  :  # do stuff with $NAME, $METHOD, $URL, $TAG
  echo "$NAME:$METHOD:$URL:$TAG"
done < <(awk '(/^#/ || NF == 0) {next};
              NF != 4 {
                printf "Error on %s:%s\n", FILENAME, NR > "/dev/stderr";
                exit 1
               };
              {print}' input.txt)

# getting the exit code from the <(...) requires bash 4.4 or newer:
wait $!

if [ "$?" -ne 0 ] ; then
 : # something went wrong in the process substitution, deal with it
fi

# By default, array var $COPROC holds the co-process' stdout and
# stdin file descriptors.   See `help coproc`.
coproc {
  awk '(/^#/ || NF == 0) {next};
       NF != 4 {
         printf "Error on %s:%s\n", FILENAME, NR > "/dev/stderr";
         exit 1
       };
       {print}' input.txt
}
awkpid="$!"
#declare -p COPROC # uncomment to see the FDs

while read -r NAME METHOD URL TAG ; do
  echo "$NAME:$METHOD:$URL:$TAG"
done <&"${COPROC[0]}"

wait "$awkpid"
echo "$?"

while read -r NAME METHOD URL TAG ; do
  :  # do stuff with $NAME, $METHOD, $URL, $TAG
  echo "$NAME:$METHOD:$URL:$TAG"
done < <(awk '(/^#/ || NF == 0) {next};
              NF != 4 {
                printf "%s:%s:Wrong number of fields\n", FILENAME, NR > "/dev/stderr";
                exit 1
               };
              {print}' input.txt)

# getting the exit code from the <(...) requires bash 4.4 or newer:
wait $!

if [ "$?" -ne 0 ] ; then
 : # something went wrong in the process substitution, deal with it
fi

# By default, array var $COPROC holds the co-process' stdout and
# stdin file descriptors.   See `help coproc`.
coproc {
  awk '(/^#/ || NF == 0) {next};
       NF != 4 {
         printf "%s:%s:Wrong number of fields\n", FILENAME, NR > "/dev/stderr";
         exit 1
       };
       {print}' input.txt
}
awkpid="$!"
#declare -p COPROC # uncomment to see the FDs

while read -r NAME METHOD URL TAG ; do
  echo "$NAME:$METHOD:$URL:$TAG"
done <&"${COPROC[0]}"

wait "$awkpid"
echo "$?"

awk '/^#/ {next}; NF == 0 {next}; NF != 4 {exit 1} ; {print}' | 
    while read NAME METHOD URL TAG ; do
        :  # do stuff with $NAME, $METHOD, $URL, $TAG
        echo "$NAME:$METHOD:$URL:$TAG"
    done

if [ "$PIPESTATUS" -eq 1 ] ; then
    : # do something to handle awk's exit code
fi

$ cat input.txt 
# comment
NAME METHOD URL TAG
a b c d
1 2 3 4
x y z
a b c d

$ ./testawk.sh <input.txt 
NAME:METHOD:URL:TAG
a:b:c:d
1:2:3:4

Note that it correctly exits on the fifth x y z input line.

awk '/^#/ {next}; NF == 0 {next}; NF != 4 {exit 1} ; {print}' | 
    while read -r NAME METHOD URL TAG ; do
        :  # do stuff with $NAME, $METHOD, $URL, $TAG
        echo "$NAME:$METHOD:$URL:$TAG"
    done

if [ "$PIPESTATUS" -eq 1 ] ; then
    : # do something to handle awk's exit code
fi

$ cat input.txt 
# comment
NAME METHOD URL TAG
a b c d
1 2 3 4
x y z
a b c d

$ ./testawk.sh input.txt 
NAME:METHOD:URL:TAG
a:b:c:d
1:2:3:4

Note that it correctly exits on the fifth x y z input line.

It's worth pointing out that because the while loop is the target of a pipe, it executes in a sub-shell and is therefore unable to alter the environment (including environment variables) of its parent script.

If that is required, then don't use a pipe, use redirection and process substitution instead:

while read -r NAME METHOD URL TAG ; do
  :  # do stuff with $NAME, $METHOD, $URL, $TAG
  echo "$NAME:$METHOD:$URL:$TAG"
done < <(awk '(/^#/ || NF == 0) {next};
              NF != 4 {
                printf "Error on %s:%s\n", FILENAME, NR > "/dev/stderr";
                exit 1
               };
              {print}' input.txt)

# getting the exit code from the <(...) requires bash 4.4 or newer:
wait $!

if [ "$?" -ne 0 ] ; then
 : # something went wrong in the process substitution, deal with it
fi

Alternatively, you can use the coproc built-in to run the awk script in the background as a co-process:

# By default, array var $COPROC holds the co-process' stdout and
# stdin file descriptors.   See `help coproc`.
coproc {
  awk '(/^#/ || NF == 0) {next};
       NF != 4 {
         printf "Error on %s:%s\n", FILENAME, NR > "/dev/stderr";
         exit 1
       };
       {print}' input.txt
}
awkpid="$!"
#declare -p COPROC # uncomment to see the FDs

while read -r NAME METHOD URL TAG ; do
  echo "$NAME:$METHOD:$URL:$TAG"
done <&"${COPROC[0]}"

wait "$awkpid"
echo "$?"

You may be able to do what you want by piping awk's output into a while read loop. For example:

awk '/^#/ {next}; NF == 0 {next}; NF != 4 {exit 1} ; {print}' | 
    while read NAME METHOD URL TAG ; do
        :  # do stuff with $NAME, $METHOD, $URL, $TAG
        echo "$NAME:$METHOD:$URL:$TAG"
    done

if [ "$PIPESTATUS" -eq 1 ] ; then
    : # do something to handle awk's exit code
fi

Tested with:

$ cat input.txt 
# comment
NAME METHOD URL TAG
a b c d
1 2 3 4
x y z
a b c d

$ ./testawk.sh <input.txt 
NAME:METHOD:URL:TAG
a:b:c:d
1:2:3:4

Note that it correctly exits on the fifth x y z input line.

The one thing I haven't done is to handle awk's exit code. Probably have to write a trap handler.

You may be able to do what you want by piping awk's output into a while read loop. For example:

awk '/^#/ {next}; NF == 0 {next}; NF != 4 {exit 1} ; {print}' | 
    while read NAME METHOD URL TAG ; do
        :  # do stuff with $NAME, $METHOD, $URL, $TAG
        echo "$NAME:$METHOD:$URL:$TAG"
    done

if [ "$PIPESTATUS" -eq 1 ] ; then
    : # do something to handle awk's exit code
fi

Tested with:

$ cat input.txt 
# comment
NAME METHOD URL TAG
a b c d
1 2 3 4
x y z
a b c d

$ ./testawk.sh <input.txt 
NAME:METHOD:URL:TAG
a:b:c:d
1:2:3:4

Note that it correctly exits on the fifth x y z input line.